Ship

SHIP. The generic name (A. S. scip, Ger. Schiff, Gr. σκάφος, from the root skap, cf. “scoop ”) for the invention by which man has contrived to convey himself and his goods upon water points in its derivation to the fundamental conception by which, when realized, a means of flotation was obtained superior to the raft, which we may consider the earliest and most elementary form of vessel. The trunk of a tree hollowed out, whether by fire or by such primitive tools as are fashioned and used with singular patience and dexterity by savage races, represents the first effort to obtain flotation depending on something other than the mere buoyancy of the material. The poets, with characteristic insight, have fastened upon these points. Homer’s hero Ulysses is instructed to make a raft with a raised platform upon it, and selects trees “withered of old, exceeding dry, that might float lightly for him” (Od ., v. 240). Virgil, glorifying the dawn and early progress of the arts, tells us, “Rivers then first the hollowed alders felt” (Georg., i. 136, ii. 451). Alder is a heavy wood and not fit for rafts. But to make for the first time a dug-out canoe of alder, and so to secure its flotation, would be a triumph of primitive art, and thus the poet’s expression represents a great step in the history of the invention of the ship.

Primitive efforts in this direction may be classified in the following order: (1) rafts—floating logs, or bundles of brushwood or reeds or rushes tied together; (2) dug-outs—hollowed trees; (3) canoes of bark, or of skin stretched on framework or inflated skins (balsas); (4) canoes or boats of pieces of wood stitched or fastened together with sinews or thongs or fibres of vegetable growth; (5) vessels of planks, stitched or bolted together with inserted ribs and decks or half decks; (6) vessels of which the framework is first set up, and the planking of the hull nailed on to them subsequently. All these in their primitive forms have survived, in various parts of the world, with different modifications marking progress in civilization. Climatic influences and racial peculiarities have imparted to them their specific characteristics, and, combined with the available choice of materials, have determined the particular type in use in each locality. Thus on the north-west coast of Australia is found the single log of buoyant wood, not hollowed out but pointed at the ends. Rafts of reeds are also found on the Australian coast. In New Guinea catamarans of three or more logs lashed together with rattan are the commonest vessel, and similar forms appear on the Madras coast and throughout the Asiatic islands. On the coast of Peru rafts made of a very buoyant wood are in use, some of them as much as 70 feet long and 20 feet broad; these are navigated with a sail, and, by an ingenious system of centre boards, let down either fore or aft between the lines of the timbers, can be made to tack. The sea-going raft is often fitted with a platform so as to protect the goods and persons carried from the wash of the sea. Upright timbers fixed upon the logs forming the raft support a kind of deck, which in turn is itself fenced in and covered over. Thus the idea of a deck, and that of side planking to raise the freight above the level of the water and to save it from getting wet, are among the earliest typical expedients which have found their development in the progress of the art of shipbuilding.

Whether the observation of shells floating on the water, or of split reeds, or, as some have fancied, the nautilus, first suggested the idea of hollowing out the trunk of a tree, the practice ascends to a very remote antiquity in the history of man. Dug-out canoes of a single tree have been found associated with objects of the Stone Age among the ancient Swiss lake dwellings ; nor are specimens of the same class wanting from the bogs of Ireland and the estuaries of England and Scotland, some obtained from the depth of 25 feet below the surface of the soil. The hollowed trunk itself may have suggested the use of the bark as a means of flotation. But, whatever may have been the origin of the bark canoe, its construction is a step onwards in the art of shipbuilding. For the lightness and pliability of the material necessitated the invention of some internal framework, so as to keep the sides apart, and to give the stiffness required both for purposes of propulsion and the carrying of its freight. Similarly, in countries where suitable timber was not to be found, the use of skins or other water-tight material, such as felt or canvas, covered with pitch, giving flotation, demanded also a framework to keep them distended and to bear the weight they had to carry. In the framework we have the rudimentary ship, with longitudinal bottom timbers, and ribs, and cross-pieces, imparting the requisite stiffness to the covering material. Bark canoes are found in Australia, but the American continent is their true home. In northern regions skin or woven material made water-tight supplies the place of bark.

The next step in the construction of vessels was the building up of canoes or boats by fastening pieces of wood together in a suitable form. Some of these canoes, and probably the earliest in type, are tied or stitched together with thongs or cords. The Madras surf boats are perhaps the most familiar example of this type, which, however, is found in the Straits of Magellan and in Central Africa (on the Victoria Nyanza), in the Malay Archipelago and in many islands of the Pacific. Some of these canoes show a great advance in the art of construction, being built up of pieces fitted together with ridges on their inner sides, through which the fastenings are passed. These canoes have the advantage of elasticity, which gives them ease in a seaway, and a comparative immunity where ordinary boats would not hold together. In these cases the body of the canoe is constructed first and built to the shape intended, the ribs being inserted afterwards, and attached to the sides, and having for their main function the uniting of the deck and cross-pieces with the body of the canoe. Vessels thus stitched together, and with an inserted framework, have from a very early time been constructed in the Eastern seas far exceeding in size anything that would be called a canoe, and in some cases attaining to 200 tons burthen.

From the stitched form the next step onwards is to fasten the materials out of which the Hull is built up by pegs or treenails; and of this system early types appear among the Polynesian islands and in the Nile boats described by Herodotus (ii. 96), the prototype of the modern “nuggur.” The raft of Ulysses described by Homer presents the same detail of construction. It is remarkable that some of the early types of boats belonging to the North Sea present an intermediate method, in which the planks are fastened together with pins or trenails, but are attached to the ribs by cords passing through holes in the ribs and corresponding holes bored through ledges cut on the inner side of each plank.

We thus arrive, in tracing primitive efforts in the art of ship construction, at a stage from which the transition to the practice of setting up the framework of ribs fastened to a timber keel laid lengthwise, and subsequently attaching the planking of the hull, was comparatively simple. The keel of the modern vessel may be said to have its prototype in the single log which was the parent of the dug-out. The side planking of the vessel, which has an earlier parentage than the ribs, may be traced to the attempt to fence in the platforms upon the sea-going rafts, and to the planks fastened on to the sides of dug-out canoes so as to give them a raised gunwale. The ribs of the modern vessel are the development of the framework originally inserted after the completion of the hull of the canoe or built-up boat, but with the difference that they are now prior in the order of fabrication. In a word, the skeleton of the hull is now first built up, and the skin, &c., adjusted to it; whereas in the earlier types of wooden vessels the outside hull was first constructed, and the ribs, &c., added afterwards. It is noticeable that the invention of the outrigger and weather platform, the use of which is at the present time distributed from the Andaman Islands eastward throughout the whole of the South Pacific, has never made its way into the Western seas. It is strange that Egyptian enterprise, which seems at a very early period to have penetrated eastward down the Red Sea and round the coasts of Arabia towards India, should not have brought it to the Nile, and that the Phoenicians, who, if the legend of their migration from the shores of the Persian Gulf to the coast of Canaan be accepted, would in all probability, in their maritime expeditions, have had opportunities of seeing it, did not introduce it to the Mediterranean. That they did not do so, if they saw it at all, would tend to prove that even in that remote antiquity both nations possessed the art of constructing vessels of a type superior to the outrigger canoes, both in speed and in carrying power.

The earliest representations that we have as yet of Egyptian vessels carry us back, according to the best authorities, to a period little short of 3000 years before Christ. Some of these are of considerable size, as is shown by the number of rowers, and by the cargo consisting in many cases of cattle. The earliest of all presents us with the peculiar mast of two pieces, stepped apart but joined at the top. In some the masts are shown lowered and laid along a high spar-deck. The larger vessels show on one side as many as twenty-one or twenty-two and in one case twenty-six oars, besides four or five steering. They show considerable camber, the two ends rising in a curved line which in some instances ends in a point, and in others is curved back and over at the stern and terminates in an ornamentation, very frequently of the familiar lotus pattern. At the bow the stem is sometimes seen to rise perpendicularly, forming a kind of forecastle, sometimes to curve backward and then forward again like a neck, λvhich is often finished into a figure-head representing some bird or beast or Egyptian god. On the war galleys there is frequently shown a projecting bow with a metal head attached, but well above the water. This, though no doubt used as a ram, is not identical with the beak à fleur d'eau, which we shall meet with in Phoenician and

Greek galleys. It is more on a level with the proem-bolion of the latter.

The impression as regards the build created by the drawings of the larger galleys is that of a long and somewhat wall-sided vessel with the stem and stern highly raised. The tendencies of the vessel to “hog,” or rise amidships, owing to the great weight fore and aft unsupported by the water, is corrected by a strong truss passing from stem to stern over crutches. The double mast of the earlier period seems in time to have given place to the single mast furnished with bars or rollers at the upper part, for the purpose apparently of raising or lowering the yard according to the amount of sail required. The sail in some of the galleys is shown with a bottom as well as a top yard. In the war galleys during action it is shown rolled up like a curtain with loops to the upper yard. The steering was effected by paddles, sometimes four or five in number, but generally one or two fastened either at the end of the stern or at the side, and above attached to an upright post in such a way as to allow the paddle to be worked by a tiller.

There are many remarkable details to be observed in the Egyptian vessels figured in Duemichen’s Fleet of an Egyptian Queen, and in Lepsius’s Denkmäler. The Egyptian ship, as represented from time to time in the period between 3000 and 1000 b.c., presents to us a ship proper as distinct from a large canoe or boat. It is the earliest ship of which we have cognizance. But there is a noticeable fact in connexion with Egypt which we gather from the tomb paintings to which we owe our knowledge of the Egyptian ship. It is evident from these records that there were at that same early period, inhabiting the littoral of the Mediterranean, nations who were possessed of sea-going vessels which visited the coasts of Egypt for plunder as well as for commerce, and that sea-fights were even then not uncommon. Occasionally the combination of these peoples for the purpose of attack assumed serious proportions, and we find the Pharaohs recording naval victories over combined Dardanians, Teucrians, and Mysians, and, if we accept the explanations of Egyptologists, over Pelasgians, Daunians, Oscans, and Sicilians. The Greeks, as they became familiar with the sea, followed in the same track. The legend of Helen in Egypt, as well as the numerous references in the Odyssey, point not only to the attraction that Egypt had for the maritime peoples, but also to long-established habits of navigation and the possession of an art of shipbuilding equal to the construction of sea-going craft capable of carrying a large number of men and a considerable cargo besides.

But the development of the ship and of the art of navigation clearly belongs to the Phoenicians. It is tantalizing to find that the earliest and almost the only evidence that we have of this development is to be gathered from Assyrian representations. The Assyrians were an inland people, and the navigation with which they were familiar was that of the two great rivers, Tigris and Euphrates. After the conquest of Phoenicia they had knowledge of Phoenician naval enterprise, and accordingly we find the war galley of the Phoenicians represented on the walls of the palaces unearthed by Layard and his followers in Assyrian discovery. But the date does not carry us to an earlier period than 900-800 β.c. The vessel represented is a bireme war galley which is “aphract,” that is to say, has the upper tier of rowers unprotected and exposed to view. The apertures for the lower oars are of the same character as those which appear in Egyptian ships of a much earlier date, but without oars. The artist has shown the characteristic details, though somewhat conventionally. The fish-like snout of the beak, the line of the parodus or outside gangway, the wickerwork cancelli, the shields ranged in order along the side of the bulwark, and the heads of a typical crew on deck (the πpωpevς looking out in front in the forecastle, an eπιβdτηs, two chiefs by the mast, and, aft, the κeλeυσττ∕s and κvβcpvητη). The supporting timbers of the deck are just indicated. The mast and yard and fore and back stays, with the double steering paddle, complete the picture.

But, although there can be little doubt that the Phoenicians, after the Egyptians, led the way in the development of the shipwright’s art, yet the information that we can gather concerning them is so meagre that we must go to other sources for the description of the ancient ship. The Phoenicians at an early date constructed merchant vessels capable of carrying large cargoes, and of traversing the length and breadth of the Mediterranean, perhaps even of trading to the far Cassiterides and of circumnavigating Africa. They in all probability (if not the Egyptians) invented the bireme and trireme, solving the problem by which increased oar-power and consequently speed could be obtained without any great increase in the length of the vessel.

It is, however, to the Greeks that we must turn for any detailed account of these inventions. The Homeric vessels were aphract and not even decked throughout their entire length. They carried crews averaging from fifty to a hundred and twenty men, who, we are expressly told by Thucydides, all took part in the labour of rowing, except perhaps the chiefs. The galleys do not appear to have been armed as yet with the beak, though later poets attribute this feature to the Homeric vessel. But they had great poles used in fighting, and the term employed to describe these (vavμaχa) implies a knowledge of naval warfare. The general characteristics are indicated by the epithets in use throughout the Iliad and the Odyssey. The Homeric ship is sharp (θoη) and swift (ωκcιd)∙, it is hollow (κo∣λη, yλaφvpη,μeγaκηrηζ), black, vermilion-cheeked (μυ∖τoτrapΥ∣oς), dark-prowed (κνανόπρωρος), curved (κopωvtς, aμxf>ιtλισσa), well-timbered (tvσσeλμoς), with many thwarts (πoλvζvγoς, ∈κaτoζυγoς). The stems and sterns are high, upraised, and resemble the horns of oxen (opθoκpaipcu). They present a type parallel in the history of the shipping of the Mediterranean with that of the vikings’ vessels of the North Sea.

On the vases, the earliest of which may date between 700 and 600 b.c., we find the bireme with the bows finished off into a beak shaped as the head of some sea monster, and an elevated forecastle with a bulwark evidently as a means of defence. The craft portrayed in some instances are evidently pirate vessels, and exhibit a striking contrast to the trader, the broad ship of burden (φopτ ∈υpetα), which they are overhauling. The trireme, which was developed from the bireme and became the Greek ship of war (the long ship, vas μ,ακρά, navis lonya, par excellence), dates, so far as Greek use is concerned, from about 700 b.c. according to Thucydides, having been first built at Corinth by Aminocles. The earliest sea-fight that the same author knew of he places at a somewhat later date, —664 b.c. , more than ten centuries later than some of those portrayed in the Egyptian tomb paintings.

The trireme was the war ship of Athens during her prime, and, though succeeded and in a measure superseded by the larger rates,—quadrireme, quinquereme, and so on, up to vessels of sixteen banks of oars (inhabilis prope magnitudinis),— yet, as containing in itself the principle of which the larger rates merely exhibited an expansion, a difference in degree and not in kind, has, ever since the revival of letters, concentrated upon itself the attention of the learned who were interested in such matters. The literature connected with the question of ancient ships, if collected, would fill a small library, and the greater part of it turns upon the construction of the trireme and the disposition of the rowers therein.

During the present century much light has been thrown upon the disputed points by the discovery (1834) at the Piraeus of some records of the Athenian dockyard superintendents, which have been published and admirably elucidated by Boeckh. Further researches carried out by his pupil Dr Graser, who united a practical knowledge of ships and shipbuilding with all the scholarship and industry and acumen necessary for such a task, have cleared up most of the difficulties which beset the problem, and enable us to describe with tolerable certainty the details of construction and the disposition of the rowers in the ancient ship of war.

One point it is necessary to insist on at the outset, because upon it depends the right understanding of the problem to be solved. The ancients did not employ more than one man to an oar. The method employed in mediaeval galleys is entirely alien to the ancient system. M. Jal, Admiral Fincati, Admiral Jurien de la Gravière, and a host of other authorities have all been led to erroneous views by neglect of the ancient texts which overwhelmingly establish this as an axiom of the ancient marine—“one oar one man.”

The distinction between “aphract” and “cataphract” vessels must not be overlooked in a description of the ancient vessels. The words, meaning “unfenced” and “fenced,” refer to the bulwarks which covered the upper tier of rowers from attack. In the aphract vessels these side plankings were absent and the upper tier of rowers was exposed to view from the side. Both classes of vessels had upper and lower decks, but the aphract class carried their decks on a lower level than the cataphract. The system of side planking with a view to the protection of the rowers dates from a very early period, as may be seen in some of the Egyptian representations, but among the Greeks it does not seem to have been adopted till long after the Homeric period. The Thasians are credited with the introduction of the improvement.

In describing the trireme it will be convenient to deal first with the disposition of the rowers and subsequently with the construction of the vessel itself. The object of arranging the oars in banks was to economize horizontal space and to obtain an increase in the number of oars without having to lengthen the vessel. We know from Vitruvius that the “interscalmium,” or space horizontally measured from oar to oar, was 2 cubits. This is exactly borne out by the proportions of an Attic aphract trireme, as shown on a fragment of a bas-relief found in the Acropolis. The rowers in all classes of banked vessels sat in the same vertical plane, the seats ascending in a lino obliquely towards the stern of the vessel. Thus in a trireme the thranite, or oarsman of the highest bank, was nearest the stern of the set of three to which he belonged. Next behind him and somewhat below him sat his zygite, or oarsman of the second bank; and next below and behind the zygite sat the thalamite, or oarsman of the lowest bank. The vertical distance between these seats was 2 feet, the horizontal distance about 1 foot. The horizontal distance, it is well to repeat, between each seat in the same bank was 3 feet (the seat itself about 9 inches broad). Each man had a resting place for his feet, somewhat wide apart, fixed to the bench of the . man on the row next below and in front of him. In rowing, the upper hand, as is shown in most of the representations which remain, was held with the palm turned inwards towards the body. This is accounted for by the angle at which the oar was worked. The lowest rank used the shortest oars, and the difference of the length of the oars on board was caused by the curvature of the ship’s side. Thus, looked at from within, the rowers amidship seemed to he using the longest oars, but outside the vessel, as we are expressly told, all the oar-blades of the same bank took the water in the same longitudinal line. The lowest or thalamite oar-ports were 3 feet, the zygite 4¼ feet, the thranite 5 1/2 feet above the water. Each oar-port was protected by an ascoma or leather bag, which fitted over the oar, closing the aperture against the wash of the sea without impeding the action of the oar. The oar was tied by a thong, against which it was probably rowed, which itself was attached to a thowl (σκαλμοί). The port-hole was probably oval in shape (the Egyptian and Assyrian pictures show an oblong). We know that it was large enough for a man’s head to he thrust through it.

The benches on which the rowers sat ran from the vessel’s side to timbers which, inclined at an angle of about 64° towards the ship’s stern, reached from the lower to the upper deck. These timbers were, according to Graser, called the diaphragmata. In the trireme each diaphragma supported three, in the quinquereme five, in the octireme eight, and in the famous tessera- conteres forty seats of rowers, who all belonged to the same “complexus,” though each to a different bank. In effect, when once the principle of construction had been established in the trireme, the increase to larger rates was effected, so far as the motive power was concerned, by lengthening the diaphragmata upwards, while the increase in the length of the vessel gave a greater number of rowers to each bank. The upper tiers of oarsmen exceeded in number those below, as the contraction of the sides of the vessel left less available space towards the bows.

Of the length of the oars in the trireme we have an indication in the fact that the length of supernumerary oars (πeρivtφ) rowed from the gangway above the thranites, and therefore probably slightly exceeding the thranitic oars in leugth, is given in the Attic tables as 14 feet 3 inches. The thranites were probably about 14 feet. The zygite, in proportion to the measurement, must have been 10½, the thalamite 7½ feet long. Comparing modern oars with these, we find that the longest oars used in the British navy are 18 feet. The university race is rowed with oars 12 feet 9 inches. The proportion of the loom inboard was about one third, but the oars of the rowers amidship must have been somewhat longer inboard. The size of the loom inboard preserved the necessary equilibrium. The long oars of the larger rates were weighted inboard with lead. Thus the topmost oars of the tesseraconteres, of which the length was 53 feet, were exactly balanced at the rowlock.

Let us now consider the construction of the vessel itself. In the cataphract class the lower deck was 1 foot above the waterline. Below this deck was the hold, which contained a certain amount of ballast, and through an aperture in this deck the buckets for baling were worked, entailing a labour which was constant and severe on board an ancient ship at sea. The keel (τpoπιs) appears to have had considerable camber. Under it was a strong false keel (χeλurrμ,α), very necessary for vessels that were constantly drawn up on the shore. Above the keel was the kelson (δpuoχoj∕), under which the ribs were fastened. These were so arranged as to give the necessary intervals for the oar-ports above. Above the kelson lay the upper false keel, into which the mast was stepped. The stem (στiΓpα) rose from the keel at an angle of about 70° to the water. Within was an apron (≠<xλκts), which was a strong piece of timber curved and fitting to the end of the keel and beginning of the stern-post and firmly bolted into both, thus giving solidity to the bows, which had to bear the beak and sustain the shock of ramming. The stem was carried upwards and curved generally backwards towards the forecastle and rising above it, and then curving forwards again terminated in an ornament which was called the acrostolion. The stern-post was carried up at a similar angle to the bow, and, rising high over the poop, was curved round into an ornament which was called “aplustre” (άφλαστον). But, inasmuch as the steering was effected by means of two rudders (πηδάλια), one on either side, there was no need to carry out the stern into a rudder post as with modern ships, and the stern was left therefore much more free, an advantage in respect of the manoeuvring of the ancient Greek man-of-war, the weapon being the beak or rostrum, and the power of turning quickly being of the highest importance.

Behind the “aplustre,” and curving backwards, was the “cheniscus” (χηvισκos), or goose-head, symbolizing the floating powers of the vessel. After the ribs had been set up and covered in on both sides with planking, the sides of the vessel were further strengthened by waling-pieces carried from stern to stem and meeting in front of the stern-post. These were further strengthened with additional balks of timber, the lower waling-pieces meeting about the water-level and prolonged into a sharp three-toothed spur, of which the middle tooth was the longest. This was covered with hard metal (generally bronze) and formed the beak. The whole structure of the beak projected about 10 feet beyond the sternpost. Above it, but projecting much less beyond the stern-post, was the “proembolion” (πρoeμβoλιov), or second beak, in which the prolongation of the upper set of waling-pieces met. This was generally fashioned into the figure of a ram’s head, also covered with metal; and sometimes again between this and the beak the second line of waling-pieces met in another metal boss called the πρoeμβoλis. These bosses, when a vessel was rammed, completed the work of destruction begun by the sharp beak at the water-level, giving a racking blow which caused it to heel over and so eased it off the beak, and releasing the latter before the weight of the sinking vessel could come upon it. At the point where the prolongation of the second and third waling-pieces began to converge inwards towards the stem on either side of the vessel stout catheads (iπαm'δes) projected, which were of use, not only as supports for the anchors, but also as a means of inflicting damage on the upper part of an enemy’s vessel, while protecting the side gangways of its own and the banks of oars that worked under them. The catheads were strengthened by strong balks of timber, which were firmly bolted to them under either extremity and both within and without, and ran to the ship’s side. Above the curvature of the upper waling-pieces into the πρoeμβdλιov were the cheeks of the vessel, generally painted red, and in the upper part of these the eyes (οφθαλμοί), answering to our hawse holes, through which ran the cables for the anchors. On either side the trireme, at about the level of the thranitic benches, projected a gangway (πάροδοί) supported by brackets (j8tαχα) springing from the upper waling-piece, and resting against the ribs of the vessel. This projection was of about 18 to 24 inches, which gave a space, increased to about 3 feet by the inward curve of the prolongation of the ribs to form supports for the deck, for a passage on either side of the vessel. This gangway was planked in along its outer side so as to afford protection to the seamen and marines, who could pass along its whole length without impeding the rowers. Here, in action, the sailors were posted as light-armed troops, and when needed could use the long supernumerary oars (πtpivιω) mentioned above. The ribs, prolonged upwards upon an inward curve, supported on their upper ends the cross beams (στρωτηpej) which tied the two sides of the vessel together and carried the deck. In the cataphract class these took the place of the thwarts (ζυya) which in the earlier vessels, at a lower level, yoked together the sides of the vessel, and formed also benches for the rowers to sit on, from which the latter had their name ({ ’ υγΓται), having been the uppermost tier of oarsmen in the bireme; while those who sat behind and below them in the hold of the vessel were called Θaλaμ7τaι or θaλdμaκeς (from θάλαμος). In the trireme the additional upper tier was named from the elevated bench (θρcivoς) on which they were placed (θρav7τai). On the deck were stationed the marines (lπιβdτaι), fighting men in heavy armour, few in number in the Attic trireme in its palmy days, but many in the Roman quinquereme, when the ramming tactics were antiquated, and wherever, as in the great battles in the harbour at Syracuse, land tactics took the place of the maritime skill which gave victory to the ram in the open sea. The space occupied by the rowers was termed tyκωπov. Beyond this, fore and aft, were the παp∈ξezρ0nα, or parts outside the rowers. These occupied 11 feet of the bows and 14 feet in the stern. In the fore part was the forecastle, with its raised deck, on which was stationed the πpωpeυs with his men. In the stern the decks (f∕rpzα) rose in two or three gradations, upon which was a kind of deck-house for the captain and a seat for the steerer ∖κυβepvηr-ηs), who steered by means of ropes attached to the tillers fixed in the upper part of the paddles, which, in later times at least, ran over wheels (τροχζλίαι), giving him the power of changing his vessel’s course with great rapidity. Behind the deck-house rose the flagstaff, on which was hoisted the pennant, and from which probably signals were given in the case of an admiral’s ship. On either side of the deck ran a balustrade (cancelli), which was covered for protection during action with felt (cilicium, παραρρύματα τρίχινα) or canvas (π. λeυκa). Above was stretched a strong awning of hide (κατάβλημα), as a protection against grappling irons and missiles of all kinds. In Roman vessels towers were carried up fore and aft from which darts could be showered on the enemy’s deck; the heavy corvus or boarding bridge swung suspended by a chain near the bows; and the ponderous δtλφις hung at the ends of the yards ready to fall on a vessel that came near enough alongside. But these were later inventions and for larger ships. The Attic trireme was built light for speed and for ramming purposes. Her dimensions, so far as we can gather them from the scattered notices of antiquity, were probably approximately as follows :—length of rowing space (Zyκωπov) 93 feet; bows 11 feet; stern 14 feet; total 118 feet; add 10 feet for the beak. The breadth at the water-line is calculated at 14 feet, and above at the broadest part 18 feet, exclusive of the gangways; the space between the diaphragmata mentioned above was 7 feet. The deck was 11 feet above the water-line and the draught about 8 to 9 feet. All the Attic triremes appear to have been built upon the same model, and their gear was interchangeable. The Athenians had a peculiar system of girding the ships with long cables (iπoζωμaτa), each trireme having two or more, which, passing through eyeholes in front of the stern-post, ran all round the vessel lengthwise immediately under the waling-pieces. They were fastened at the stern and tightened up with levers. These cables, by shrinking as soon as they were wet, tightened the whole fabric of the vessel, and in action, in all probability, relieved the hull from part of the shock of ramming, the strain of which would be sustained by the waling-pieces convergent in the beaks. These rope-girdles are not to be confused with the process of undergirding or trapping, such as is narrated of the vessel in which St Paul was being carried to Italy. The trireme appears to have had three masts. The mainmast carried square sails, probably two in number. The foremast and the mizen carried lateen sails. In action the Greeks did not use sails, and everything that could be lowered was stowed below. The mainmasts and larger sails were often left ashore if a conflict was expected.

The crew of the Attic trireme consisted of from 200 to 225 men in all. Of these 174 were rowers,—54 on the lower bank (thalamites), 58 on the middle bank (zygites), and 62 on the upper bank (thranites),—the upper oars being more numerous because of the contraction of the space available for the lower tiers near the bow and stern. Besides the rowers were about 10 marines (tπtβaτai) and 20 seamen. The officers were the trierarch and next to him the helmsman (κυβepvf∣τηs), who was the navigating officer of the trireme. Each tier of rowers had its captain (σroιχapχ6s). There were also the captain of the forecastle (πρωpti>s), the “keleustes” who gave the time to the rowers, and the ship’s piper (τpιηpaυ∖∙f∣s). The rowers descended into the seven-foot space between the diaphragmata and took their places in regular order, beginning with the thalami tes. The economy of space was such that, as Cicero remarks, there was not room for one man more.

The improvement made in the build of their vessels by the Corinthian and Syracusan shipwrights, by which the bows were so much strengthened that they were able to meet the Athenian attack stem on (προσβολή), caused a change of tactics, and gave an impetus to the building of larger vessels—quadriremes and quinqueremes—in which increased oar-power was available for the propulsion of the heavier weights.

In principle these vessels were only expansions of the trireme, so far as the disposition of the rowers was concerned, but the speed could not have increased in proportion to the weight, and hence arose the variety of contrivances which superseded the ramming tactics of the days of Phormio. In the century that succeeded the close of the Peloponnesian War the fashion of building big vessels became prevalent. We hear of various numbers of banks of oars up to sixteen (tκκaβtκηpη) —the big vessel of Demetrius Poliorcetes. The famous tesseraconteres or forty-banked vessel of Ptolemy Philopator was in reality nothing more than a costly and ingenious toy, and never of any practical use. The fact, however, of its construction shows the extent to which the shipwright’s art had been developed among the ancients.

The Romans, who developed their naval power during the First Punic War, were deficient in naval construction till they learnt the art from their enemies the Carthaginians. They copied a quinquereme which had drifted on to the coast, and, with crews taught to row on frames set up on dry land, manned a fleet which we are told was built in sixty days from the time the trees were cut down. After the Punic War, in which the use of boarding tactics gave the Romans command of the sea, the larger rates —quinqueremes, hexiremes, octiremes—continued in use until at Actium the fate of the big vessels was sealed by the victory of the light Liburnian galleys. The larger classes, though still employed as guardships for some time, fell into disuse, and the art of building them and the knowledge of their interior arrangements were lost. [table]

Mediaeval Ships.— It is not at present possible to trace in its successive stages the transition from the ancient ship of war to the mediaeval galley. The sailing vessels of the time of the early Roman empire, such as that in which St Paul suffered shipwreck or the great merchantman described by Lucian, were the direct precursors, not only of the mediaeval merchant vessels, but also of the large sailing vessels which, after the invention of gunpowder, and the consequent necessity of carrying marine artillery, superseded the long low galleys propelled by oars. The battle of Actium gave the death-blow to the ancient type of vessel with its many banks of oars. The light Liburnian galleys which, though fully decked, were aphract, and, according to Lucan’s testimony (bk. iii.),

Ordine contentae gemino crevisse Liburnae, had only two banks of oars, were biremes. This apparently became the type of Roman war galleys; and, though the old name trireme survived, its meaning became simply “man of war,” and did not any longer imply three banks of oars. Light vessels were in vogue, and galleys with single banks of oars are common in the representations on coins and in such frescos as survive, but trireme and quinquereme, &c., have vanished.

A cloud of obscurity rests on these, the dark ages of naval history. We know nothing of the character and composition of the fleet in which Ricimer defeated the Vandals in the 5th century of our era. Nor have we any details of the fleets of the Byzantine empire until the end of the 9th century, when a light is thrown upon the subject by the Tactica of the emperor Leo. This emperor, in giving his directions as to the constitution of his fleet, prescribes that dromones (δpo∕xωv∈s)—that is, triremes— are to be got ready in the dockyards with a view to a naval engagement. The vessels are not to be too light or too heavy. They are to be armed with siphons for the projection of Greek fire. They are to have two banks of oars, with twenty-five rowers a-piece, on each side. Some of the vessels are to be large enough to carry two hundred men; others are to be smaller, like those called galleys or one-banked vessels, swift and light (eλαττoυv δpoμικωτuτoυς oiortt γαλα(ας ?; μονή pas λeγoμivovς τaχιvoυς κai ιλa<f>povς). Here we have the name galleys distinctively attached to vessels with one bank of oars. This passage should have saved much of the labour that has been thrown away in attempting to prove that the distribution of rowers in the mediaeval galleys was upon the same principle as that observed in the ancient biremes or triremes.

The light thrown by the philosophic Byzantine on the naval construction and equipment of his time is but a passing flash. After the 9th century there is darkness again until the 11th and 12th centuries, when the features of the mediaeval galley first begin to be visible. And here perhaps it is not out of place to say that it is necessary to distinguish between those imaginary representations of the antique in which painters, such as Tintoret, give fanciful arrangement to the oars of their galleys, so as to meet their ideas of bireme or trireme, from those that are historically faithful and figure, perhaps in an ungainly and inartistic manner, the galleys of Venice and Genoa as they appeared in the Middle Ages. It would exceed the space at our disposal here to enter into details which can be gathered from Jal’s Archéologie Navale and the Glossaire Nautique of the same author, or the later works of Admiral Jurien de la Gravière and Admiral Fincati. It must suffice to indicate here a few of the main characteristics in which the mediaeval galley differs from the ancient, and exhibits the last development of man-power as applied to motion in vessels larger than the boats of the present day.

These characteristics may be sketched briefly. Upon the mediaeval galley, which was essentially a one-banked galley (μονόκροτον), the use of the longer oar or sweep took the place of the small paddling oars of the ancient vessel. The increased length of the oar requiring for its efficiency greater power than one man could employ led to the use of more than one man to an oar. The necessity therefore arose of placing the weight (or point at which the oar, used as a lever, worked against the thowl, and so pressed against the water, which is the fulcrum) at a greater distance from the force or man who moved the lever. This was gained by the invention of the apostis. Upon the hull of the mediaeval galley was laid a framework which stood out on either side from it, giving on either side a strong external timber, running parallel to the axis of the vessel, in which the thowls were fixed against which the oars were rowed. It will be readily understood how this arrangement gave a greater length inboard for the oar as compared with that of the ancient vessels, where the thowl stood in the aperture of the vessel’s side or port-hole. On the inner side, rising inwards towards the centre line of the decks and inclining upwards, were the banks or benches for the rowers, arranged à la scaloccio, who could each grasp the handle of the oar, moving forward as they depressed it for the feather, and backward for the stroke as they raised their hands for the immersion of the blade. The stroke no doubt was slower than that of the ancient galleys, but much more powerful. For the rest we must refer to the works above mentioned, where the reader will find minute descriptions of the build and the equipment of mediaeval vessels, such as those which fought at Lepanto or carried the proud ensign of the Genoese republic.

Literature.— 1. For Ancient Ships Duemichen, Fleet of an Egyptian Queen; Chabas, Etudes sur l'Antiquité Historique ; Rawlinson, Ancient Monarchies; Scheffer, De Militia Navali Veterum·, Boeckh, Urkunden über das Seewesen des Attischen Staates ; B. Graser, De Re Navali Veterum·, Id., Das Model eines Athenischen Fünfreihenschiffes (Pentere) aus der Zeit Alexanders des Grossen im Königlichen Museum zu Berlin; Id., Die Gemmen des Königlichen Museums zu Berlin mit Darstellungen antiker Schiffe·, Id., Die ältesten Schiffsdarstellungen auf antiken Münzen ; A. Cartauld, La Trière Athénienne ; Breusing, Die Nautik der Alten·, Smith, Voyage and Shipwreck of St Paul. 2. For Mediaeval Shipping:— A. Jal, Archéologie Navale and Glossaire Nautique ; Jurien de la Gravière, Derniers Jours de la Marine à Rames, Paris, 1885; Fincati, Le Triremi. (E. WA.)

WITHIN the memory of the present generation shipbuilding, like many other arts, has lost dignity by the extended use of machinery and by the subdivision of labour. Forty years ago it was still a “mystery” and a “craft.” The well-instructed shipbuilder had a store of experience on which he based his successful practice. He gained such advantages in the form and trim and rig of his vessels by small improvements, suggested by his own observation or by the traditions of his teachers, that men endeavoured to imitate him, neither he nor they knowing the natural laws on which success depended. He had also a good eye for form, and knew how to put his materials together so as to avoid all irregularity of shape on the outer surfaces, and how to form the outlines and bounding curves of the ship so that the eye might be compelled to rest lovingly upon them. He was skilled also in the qualities of timber. He knew what was likely to be free from “rends” and. “shakes” and “cups” which would cause leakage, and which would be liable to split when the bolts and treenails were driven through it. He knew what timber would bear the heat of tropical suns without undue shrinking, and how to improve its qualities by seasoning. He could foretell where and under what circumstances premature decay might be expected, and he could choose the material and adjust the surroundings so as to prevent it. He knew what wood was best able to endure rubbing and tearing on hard ground, and how it ought to be formed so that the ship might have a chance of getting off securely when she accidentally took the ground or got on shore. Such men were to be found on all the sea-coasts of Europe and on the shores of the Atlantic in America.

A great change came over the art when steam was introduced. The old proportions and forms so well suited for the speeds of the ships and for the forces impressed upon them were ill adapted for propulsion by the paddle, and still less so for propulsion by the screw. Experience had to be slowly gained afresh, for the lamp of science burned dimly. It needed to be fed by results, by long records of successes and failures, before it was able to direct advancing feet. The further change from wood to iron and then to steel almost displaced the shipwright. Ships for commercial purposes may be said to be built now, so far as their external hulls are concerned, by draftsmen and boilermakers. The centres of the shipbuilding industry have changed. The ports where oaks (Italian, English, and Dantzic), pines from America and the north of Europe, teak from Moulmein, and elm from Canada were most accessible,—these marked the suitable places for shipbuilding. The Thames was alive with the industry from Northfleet to the Pool. It still lingers, but it is slowly dying out. Travellers along the

Mediterranean shores from Nice to Genoa mark the completeness of the change which a few years have made.

The Tyne and the Clyde and the Mersey have become the principal centres of the trade. It has been drawn there because the iron and the coal are near.

But, while the art of shipbuilding has lost dignity, the science of naval construction has increased in importance. English art is of an eminently practical character. It is shy of experiment, as being costly in itself and likely to lead to delays and changes of system and of plant. It loves large orders and rapid production. It practises great subdivision of the details in order to cheapen production, and it stereotypes modes of work. There is no lack of boldness and enterprise; but the patient continuous inquiry and the slow but sure building up of theory upon research,—this is the exception. Naval construction in England has had the good fortune during the last quarter of a century to have not only a thriving industry but a home for research. Twenty-five years ago, when the high-pressure condensing engine was in its infancy, when ship building steel was not, and armour-plated ships had not yet displaced the wooden line-of-battle ship, this home was founded. The Institution of Naval Architects may be fairly called the home for research in naval construction. It owes its establishment mainly to four well-known men— John Scott Russell, Dr Joseph Woolley, Lord Hampton, for many years its honoured president, and Sir Edward Reed, its first secretary. It has published every year a volume of Transactions recording the experience of all the shipbuilders and marine engineers in England. These Transactions contain also valuable contributions from French, Italian, German, and other eminent constructors and engineers.

Shortly after the foundation of the Institution one of its members, Mr William Froude, set up an experimental establishment at Torquay, under the auspices and with the assistance of the Admiralty. The object was to submit to experiment various proportions and forms of ships in model in order to compare the relative resistances in the same model at various speeds, and in different forms and proportions at equal speeds. There was some reason to doubt the possibility of inferring from a model on a scale of 3/4 of an inch to a foot what would happen in a ship of corresponding form and proportions. In order to establish satisfactorily the relations between the real and the model ship a series of experiments was desirable upon a real ship in which the resistances could be measured by a dynamometer at various speeds and compared with those indicated by the model. Up to the date of this trial the “scale of comparison” which had been employed by Mr Fronde was based upon prima facie theoretical truth, and it had some experimental justification. It may be stated as follows, as given by Mr Froude in the volume for 1874 of the Transactions of the Institution of Naval Architects :—

If a ship be D times the “dimension,” as it is termed, of the model, and if at the speeds V 1 , V 2 , V 3 . . . the measured resistances of the model are R 1 , R 2 , R 3 . . . ., then for speeds D 1/2 V 1 , D 1/2 V 2 D 1/2 V 3 , . . . of the ship, the resistances will be D 3 R 1 , D 3 R 2 D 3 R 3 . . . To the speeds of model and ship thus related it is convenient to apply the term “corresponding speeds.” For example, suppose two similar ships, the length, breadth, depth, &c.,of which were double one of the other. Then, if at a given speed (say 10 knots) the resistance of the smaller ship were ascertained, we may infer that at a speed of √2×10=14·14 knots in the larger ship there would be a resistance 8 times as great as in the smaller vessel.

This law is in accordance with the old rule that the resistance varies as the square of the velocity, and also as the area of the surface exposed to resistance. It takes into account both the resistance due to surface friction (subject to some correction) and the formation of deadwater eddies. The passage of the ship through the water creates waves which are dependent for their character upon the proportions and form of the ship. These constitute also an element of resistance. They are due to differences of hydrodynamic pressure inherent in the system of stream-lines which the passage of the ship creates. These wave-configurations should be precisely similar when the originating forms are similar and are travelling at speeds proportional to the square roots of their respective dimensions, because the resulting forces will be in that case as the square of the speeds. For example, if the surface of the water surrounding a ship 160 feet long, travelling at 10 knots an hour, were modelled together with the ship, on any scale, the model would equally represent, on half that scale, the water surface surrounding a ship of similar form 320 feet long, travelling at 14·14 knots an hour; or again, on 16 times that scale, the water surface surrounding a model of the ship 10 feet long, travelling at 2 1/2 knots. Experiment has abundantly confirmed this proportion as to the similarity of waves caused by similar forms travelling at corresponding speeds. The resistance caused to these forms respectively by the development of the waves would therefore also be proportionate to the cubes of the dimensions of the forms and would follow the law of comparison stated above. It is necessary, however, to observe that, in dealing with surfaces having so great a disparity in length and speed as those of a model and of a ship, a very tangible correction is necessary in regard to surface friction.

The vessel tried by Mr Froude for confirming the law of comparison was H. M. S. “Greyhound,” of 1157 tons. She was towed by H.M.S. “Active,” of 3078 tons, from the end of a boom 45 feet long, so as to avoid interferences of “wake.” It was found to be possible to tow up to a speed of nearly 13 knots. The actual amount of towing strain for the “Greyhound” was approximately as follows:—at 4 knots, 0·6 ton; at 6, 1·4 tons; at 8, 2·5 tons; at 10, 4·7 tons; and at 12, 9·0 tons.

Comparing the indicated horse-power of the “Greyhound” when on her steam trials and the resistance of the ship as determined by the dynamometer, it appears that, making allowance for the slip of the screw, which is a legitimate expenditure of power, only about 45 per cent. of the power exerted by the steam is usefully employed in propelling the ship, and that the remainder is wasted in friction of engines and screw and in the detrimental reaction of the propeller on the stream lines of the water closing in around the stern of the vessel.

We may describe in Mr Froude’s own words the system of experiment now regularly carried out for the Admiralty, a system which has been successfully copied in other countries and also by a private shipbuilding firm, Messrs Denny of Dumbarton :—

“That system of experiments involves the construction of models of various forms (they are really fair-sized boats of from 10 to 25 feet in length), and the testing by a dynamometer of the resistances they experienced when running at various assigned appropriate speeds. The system may be described as that of determining the scale of resistance of a model of any given form, and from that the resistance of a ship of any given form, rather than as that of searching for the best form, and this method was preferred as the more general, and because the form which is best adapted to any given circumstances comes out incidentally from a comparison of the various results. We drive each model through the water at the successive assigned appropriate speeds by an extremely sensitive dynamometrical apparatus, which gives us in every case an accurate automatic record of the model’s resistance, as well as a record of the speed. We thus obtain for each model a series of speeds and the corresponding resistances; and, to render these results as intelligible as possible, we represent them graphically in each case in a form which we call the ‘ curve of the resistance’ for the particular model. On a straight base line which represents speed to scale we mark off the series of points denoting the several speeds employed in the experiments, and at each of these points we plant an ordinate which represents to scale the corresponding resistance. Through the points defined by these ordinates we draw a fair curved line, and this curve constitutes what I have called the curve of resistance. This curve, whatever be its features, expresses for the model of that particular form what is in fact and apart from all theory the law of its resistance' in terms of its speed; and what we have to do is if possible to find a rational interpretation of the law. Now we can at once carry the interpretation a considerable way; for we know that the model has so many square feet of skin in its surface, and we know by independent experiments how much force it takes to draw a square foot of such skin through the water at each individual speed. The law is very nearly—and for present convenience we may speak as if it were exactly—that skin resistance is as the area simply, and as the square of the speed. Now, we have so many square feet of immersed skin in the model, and the total skin resistance is a certain known multiple of the product of that number of square feet and of the square of the speed. Now, when we lay off on the curve of resistance a second curve which represents that essential and primary portion of the resistance, then we find this to bo the result: the curve of skin resistance when drawn is found to be almost identical with the curve of total resistance at the lower speeds; but as the speed is increased the curve of total resistance is found to ascend more or less, and in some cases to ascend very much above the curve of skin resistance. The identity of the two curves at the lower speeds is the practical representation of a proposition which the highest mathematicians have long been aware of, and which I have lately endeavoured to draw the public attention to, and to render popularly intelligible, namely, that when a ship of tolerably fine lines is moving at a moderate speed the whole resistance consists of surface friction. The old idea that the resistance of a ship consists essentially of the force employed in driving the water out of her way, and closing it up behind her, or, as it has sometimes been expressed, in excavating a channel through the track of water which she traverses,—this old idea has ceased to be tenable as a real proposition, though prima facie we know that it was an extremely natural one. Wo now know that, at small speeds, practically the whole resistance consists of surface friction, and some derivative effects of surface friction, namely, the formation of frictional eddies, which is due to the thickness of the stem and of the sternpost; but this collateral form of frictional action is insignificant in its amount unless the features of the ship in which it originates are so abruptly shaped as to constitute a departure from that necessary fineness of lines which I have described; and we do not attempt to take an exact separate account of it. Thus we divide the forces represented by the curve of resistance into two elements,—one ‘skin resistance,’ the other which only comes into existence as the speed is increased, and which we may term ‘ residuary resistance. ’ And we have next to seek for the cause and governing laws of this latter element. Now when the passage of the model along the surface of the water is carefully studied, we observe that the special additional circumstance which becomes apparent as the speed is increased is the train of waves which she puts in motion; and indeed it has long been known that this circumstance has important bearings on the growth of resistance. It is in fact certain that the constant formation of a given series involves the operation of a constant force, and the expenditure of a definite amount of power, depending on the magnitude of those waves and the speed of the model; and, as we thus naturally conclude that the excess of resistance beyond that due to the surface friction consists of the force employed in wave-making, we in a rough way call that residuary resistance ‘wave-making resistance.’

“Perhaps I had better say a few words more about the nature and character of these waves. The inevitably widening form of the ship at her ‘ entrance ’ throws off on each side a local oblique wave of greater or less size according to the speed and to the obtuseness of the wedge, and these waves form themselves into a series of diverging crests, such as we are all familiar with. These waves have peculiar properties. They retain their identical size for a very great distance with but little reduction in magnitude. But the main point is that they become at once dissociated from the model, and after becoming fully formed at the bow, they pass clear away into the distant water and produce no further effect on her resistance. But, besides those diverging waves, there is produced by the motion of the model another notable series of waves which carry their crests transversely to her line of motion. Those waves, when carefully observed, provo to have the form shown in detail in fig. 1. In the figure there is shown the form of a model which has a long parallel middle body accompanied by the series of these transverse waves as they appear at some one particular speed with the profile of the series defined against the side of the model; only I should mention that for the sake of distinctness the vertical scale of the waves has been made double the horizontal scale, so that they appear relatively to the model about twice as high as they really are. The profile is drawn from exact and careful measurements of the actual wave features as seen against the side of the model. It is seen that the wave is largest where its crest first appears at the bow, and it reappears again and again as we proceed sternwards along the straight side of the model, but with successively reduced dimensions at each reappearance. That reduction arises thus :—in proportion as each individual wave has been longer in existence, its outer end has spread itself farther into the undisturbed water on either side, and, as the total energies of the wave remain the same, the local energy is less and less, and the wave-crest, as viewed against the side of the ship, is constantly diminishing. We see the wave-crest is almost at right angles to the ship, but the outer end is slightly deflected sternward from the circumstance that when a wave is entering undisturbed water its progress is a little retarded, and it has to deflect itself into an oblique position, so that its oblique progress shall enable it exactly to keep pace with the ship. The whole wave-making resistance is the resistance expended in generating first the diverging bow waves, which, as we have seen, cease to act on the ship when once they have rolled clear of the bow; secondly, these transverse waves, the crests of which remain in contact with the ship’s side; and thirdly, the terminal wave, which appears independently at the stern of the ship. This latter wave arises from causes similar to those which create the bow wave, namely, the pressure of the streams which, forced into divergence then, here converge under the run of the vessel, and re-establish an excess of pressure at their meeting. The term ‘ wave-making resistance ' represents, then, the excess of resistance beyond that due to surface friction, and that excess we know to be chiefly due to this formation of waves by the ship.”

Pursuing these experiments it was found that not only was there a certain length of form necessary in a ship designed to attain a certain speed economically,—a fact which Mr Scott Russell did much to establish,—but that there was also a considerable increase in wave-making resistance dependent upon the position of the afterbody or run of the ship with reference to the wave-system left by the bow. Stating this again in Mr Froude’s words :—

“The waves generated by the ship in passing through the water originate in the local differences of pressure caused in the surrounding water by the vessel passing through it; let us suppose, then, that the features of a particular form are such that these differences of pressure tend to produce a variation in the water level shaped just like a natural wave, or like portions of a natural wave of a certain length.

“Now an ocean wave of a certain length has a certain appropriate speed at which only it naturally travels, just as a pendulum of a certain length has a certain appropriate period of swing natural to it. And, just as a small force recurring at intervals corresponding to the natural period of swing of a pendulum will sustain a very large oscillation, so, when a ship is travelling at the speed naturally appropriate to the waves which its features tend to form, the stream line forces will sustain a very large wave. The result of this phenomenon is, that as a ship approaches this speed the waves become of exaggerated size, and run away with a proportionately exaggerated amount of power, causing corresponding resistance. This is the cause of that very disproportionate increase of resistance experienced with a small increase of speed when once a certain speed is reached.

“We thus see that the speed at which the rapid growth of resistance will commence is a speed somewhat less than that appropriate to the length of the wave which the ship tends to form. Now, the greater the length of a wave is the higher is the speed appropriate to it; therefore the greater the length of the waves which the ship tends to form the higher will be the speed at which the wave-making resistance begins to become formidable. We may therefore accept it as an approximate principle that the longer are the features of a ship which tend to make waves the higher will be the speed she will be able to go before she begins to experience great wave-making resistance, and the less will be her wave-making resistance at any given speed. This principle is the explanation of the extreme importance of having at least a certain length of form in a ship intended to attain a certain speed; for it is necessary, in order to avoid great wave-making resistance, that the ‘wave features,’ as we may term them, should be long in comparison with the length of the wave which would naturally travel at the speed intended for the ship.

“This view of the matter, then, recognizes the tendency of a ship, when the speed bears a certain relation to the length of. her wave-making features, to make largo waves and to incur corresponding wave-making resistance. But it does not take account of the possibility of the waves made by one feature of the form so placing themselves with reference to other features as, by the differences of pressure essential to their existence, either to cause an additional resistance, or on the other hand to cause a forward force which partly counterbalances the resistance originally due to their creation. The way in which this may occur we have seen strikingly exhibited in the results of the experiments I have been describing. We see that in the very long parallel-sided form the sternmost of the train of waves left by the bow has become so small that its effect on the stern is almost insensible; and here we find, consequently, the united resistance due simply to the generation of a separate wave-system by each end of the ship. As we gradually reduce the length of middle-body, the stern is brought within the reach of waves large enough to produce a sensible effect, and according as it is brought into conjunction with a crest or hollow, the total wave-making resistance becoming least of all (except at the very highest speed) when the middle-body is reduced to nothing.”

The variations in residuary resistance due to these transverse wave-formations are variations of quasi-hydrostatic pressure against the after-body, corresponding with the changes in its position with reference to the phases of the train of waves, there being a comparative excess of pressure (causing a forward force or diminution of resistance) when the after-body is opposite a crest, and the reverse when it is opposite a trough.

It may be proper to introduce here some remarks as to the stream lines which have been referred to in the foregoing considerations. The statement of the case as given by Mr Froude, and derived by him mainly from the investigations of Prof. Rankine, is as follows :—

“By a ‘perfect fluid’ is meant one the displacements of which are governed solely by the laws expressed in the equation of fluid motion, the particles of which therefore are without viscosity, and are capable of gliding rectilinearly along a perfectly smooth surface or past each other without frictional interference. By an imperfect fluid is meant one in which, as in water, as well as those with which we are practically acquainted, such frictional interference is inevitable.

“Dealing first, then, with the case of steady rectilinear motion in a perfect incompressible fluid, infinitely extended in all directions, it is plain that the motion will create differences of pressure, and therefore changes of velocity, in the particles of the surrounding fluid, which thus move in what are called ‘stream lines.’ At the commencement of the motion of the body the particles of the fluid undergo acceleration in their respective stream-line paths, and these accelerations imply a resistance experienced by the body; but after the motion has become established the differences of pressure satisfy themselves by keeping up the stream-line configuration; the energy which the particles receive from the body while they are being pushed aside by it along their stream-line paths is finally redelivered by them to it as they collapse around it, and come to rest after its passage, and the integrals of the + and - pressures on the body are exactly equal at every moment. The manner in which this is effected is governed by the general laws of fluid motion, as expressed by the well-known equations; and, since these equations contain no term which implies a loss of energy, the energy existing in the body, as well as in the streamline system, remains unaltered; so that, if the motion is steady, or without acceleration or retardation, the body passes through this theoretically perfect fluid absolutely without resistance. Nor must it be thought a paradox (for it is unquestionable) that even a plane moving steadily at right angles to itself through a perfect fluid would in the manner described experience no resistance. But if the fluid, instead of being infinite in all directions, be bounded by a definite free surface parallel to the line of motion, such as a water level, the existence of this surface cuts off the reactions of all those particles which would have existed beyond the surface had the fluid been unlimited alike in all directions, and which would have given back in the manner described the energy imparted to them. By the absence of these reactions the stream-line motions which would have existed in the infinite fluid are modified, and the differences of pressure involve corresponding local elevations of the surface of the water in the vicinity of the moving body. And since, in consequence of the action of gravitation (the force which controls the surface), a water protuberance seeks immediately to disperse itself into the surrounding fluid in accordance with the laws of wave motion, the local elevation partly discharges itself along the surface by waves which carry with them the amount of energy embodied in their production. This energy is, in fact, part of the aggregate energy which was imparted to the particles of fluid while they were being pushed aside, and which, in the infinitely extended fluid, would have been wholly restored to the body during their collapse after its passage, but is now, in fact, dissipated. The exact equality between the + and - pressures no longer exists, and the body experiences a definite resistance which it would not do if the fluid were infinite in all directions.

“It is clear, moreover, that the nearer the moving body approaches the surface the greater are the differences of pressure to be satisfied, the greater will be the waves formed, and the greater the dissipation of energy. Thus, for example, a fish will experience an increase of resistance as its path lies nearer to the surface, the train of waves it creates becoming then a visible accompaniment of its progress. A fortiori, when the body moves along the surface as a ship does on water, those differences of pressure which would exist during the motion if the fluid were infinite in all directions satisfy themselves in still larger waves, which, in fact, are the waves which accompany the body in its motion. The waves which thus visibly accompany a vessel in transitu form a marked phenomenon in river steaming. Thus we see how, although in a perfect fluid extended infinitely in all directions, a body, when once put in motion, would move absolutely without resistance, yet, when the fluid is bounded by a gravitating surface at or near the line of motion, the body will experience resistance by the formation of waves, notwithstanding that the fluid is a perfect one.

“If the fluid is again supposed to be infinite in all directions, but imperfect, the phenomena previously described undergo appropriate modifications, and the moving body will also suffer a specific resistance,—in the first place by its having to overcome the friction and viscosity of those particles of the fluid with which it is in contact, and next because the friction of the surrounding particles inter se destroys that orderly arrangement of the stream-line configuration which allows of the energy imparted to the particles being returned without loss. If the supposed imperfect fluid is bounded by a free surface, as already described, and the body moves at or near this surface, it will experience resistances depending on fluid friction, almost exactly in the same manner as if the fluid were infinite in all directions. It will also experience very nearly the same resistance in virtue of the wave-making action as in the perfect fluid; and we here see the two sources of resistance existing independently of each other, and due to totally different causes.

Important as the question is as to the effect of form upon resistance, that of its effect upon stability or steadiness at sea is even more so. Before the use of steam for the propulsion of ships the speed which could be attained in seagoing ships by sail power was largely a question of stability or power to carry a large spread of canvas without inclining or “heeling” too greatly. Small differences in the form of the transverse sections of the ship in the region of the load water-line and under water were influential in this respect, and naval constructors occupied themselves greatly with such questions. The form of the problem completely changes when the propelling power is no longer an upsetting force. The important questions in steam ships are the proportions of length, breadth, and depth; the form of “entrance’’ and “run”; the construction of propelling machinery within the ship; and the proportions, form, and number of revolutions of the propeller. But, while this is so, the effect of the stability of the steamship upon her behaviour at sea, as a question of rolling or “labouring,” remains very great. There are, moreover, a very large number of seagoing ships still dependent upon sails for their propulsion, and the question of sailing power is very important in vessels employed on our coasts for commerce and for pleasure. The latest and most complete investigation of questions of stability is to be found in Sir Edward J. Reed’s recently published work, The Stability of Ships. There is a more popular exposition of the subject by Mr W. H. White, director of naval construction, in his Manual of Naval Architecture (1877, 2d ed. 1882), of which use has been made in the following pages.

A ship floating freely and at rest in still water displaces a volume of water exactly equal in weight to her own weight. The circumstances of the water in which she floats are in fact the same whether the cavity made in the water by the ship is filled by the ship as in fig. 2, or by a volume of water having the same weight as the ship (fig. 3).

When the ship oc cupies the cavity the whole of her weight may be sup posed to be con centrated at her centre of gravity,

G, fig. 2, and to act vertically downwards. When the cavity is filled with water its weight, called in relation to the ship the “displacement,” may be supposed to be concentrated at B, fig. 3, which is the centre of gravity of the “displacement” or of the displaced water. This centre of gravity is usually known in relation to the ship as the “centre of buoyancy.” The weight of this water may be supposed to be concentrated at B, and to act vertically downwards. As this water would remain in the cavity at rest, its downward' pressure must be balanced by equal upward pressures, that is by the buoyancy of the surrounding water. These upward pressures must act in the same way as if there were a single pressure equal and opposite to the weight of the water, and acting through the “centre of buoyancy." In fig. 2 a ship is represented floating freely and at rest in still water. Her total weight may be supposed to act vertically downwards through the centre of gravity G, and the buoyancy vertically upwards through the centre of buoyancy. The second condition which the ship floating freely and at rest in still water will always satisfy is therefore said to be that her centre of gravity will lie in the same vertical line with the centre of gravity of the volume of water which she displaces. So long as the ship rests under the action of these opposing and balanced forces the line joining the centres B and G is vertical and represents the common line of action of the weight and buoyancy. There are of course horizontal fluid pressures acting upon her, but these are balanced among themselves.

The ship may be floating at rest, but under constraint, and not freely. There may be the pressure of wind on the sails, or the strain of a rope holding her in a position of rest although the centres B and G are no longer in the same vertical line. Fig. 4 represents such a case.

The vessel is at rest, but there is some ex-

ternal force operating other than that of buoyancy; and the equal and opposite forces of the weight and buoyancy act in different vertical lines, and no longer balance each other. They form a mechanical

“couple,” tending to move the ship from the position of con strained rest in which she is shown. If W

represents the total weight of the ship (in tons), and d the per pendicular distance between the parallel lines of action of the weight and buoyancy (in feet), then the operative moment of the “couple” is represented by the product of the two quantities W and d, measured in foot-tons. If the constraint is removed, and the vessel is freed from all external forces save those of the fluid in which she floats, she will move under the operation of the “couple” towards the upright position until the consequent alteration in the form of the cavity of the displacement brings the centre of buoyancy into the same vertical with the centre of gravity of the ship. What has been illustrated by reference to transverse inclination of the ship is equally true of oblique or longitudinal inclinations. If the position of the weights in the ship remains unaltered under such changes of inclination the centre of gravity remains unaltered. In all calculations it has to be assumed that the centre of gravity is a fixed point in the ship, and that movable weights will be secured in the ship. With this assumption the position of the centre of gravity of a ship can be correctly assigned by calculation, small disturbances caused by movements of men, &c., not being large enough to be appreciable.

The statical stability of a ship may be defined as the effort which she makes when inclined steadily by external forces to overcome the constraint and return to the position in which she floats freely, at or near the upright. This effort, as already explained, depends upon the position of the centre of buoyancy B, or the distance from the vertical line through G which the altered form of the cavity of the displacement has caused it to assume. It may always be measured by the product of the two quantities W (in tons) and d (in feet) (see fig. 4). This product in foot-tons is known as the “moment of statical stability” for the particular angle of inclination and corresponding position of B which are assumed. A little reflexion will show that when large angles of inclination are reached the centre B ceases to recede from the vertical through the centre of gravity of the ship, but will, as the inclination increases, approach this vertical line, and eventually pass to the other side of it.

The moment of statical stability is at its maximum when the distance d is greatest. The angle which the ship has reached when the centre B has reached this point is called the “angle of maximum stability.” As the centre B travels backwards from this position with the increasing inclination of the ship the distance d decreases and the righting power of the ship decreases proportionately. When B passes the vertical line through G the moment of stability changes its character and becomes an upsetting force, which will continue to act until the ship reaches a new position of rest, usually bottom upwards. The angle which the ship reaches before this change takes place, i.e., when B passes to the other side of the vertical line through G, is called the “angle of vanishing stability” and it indicates the ship’s “range of stability.” The change may occur at very small angles if the ship is crank and her sides are low in the water. It may not and sometimes does not occur, on the other hand, until the ship is lying on her beam ends.

If a curve is plotted out showing these positions and indicating also how d first increases and then decreases as the ship is inclined more and more from the upright, the curve is known as the curve of stability. A “stiff ship” is one which opposes great resist ance to inclination from the upright when under sail or acted upon by external forces. A “crank ship” is one very easily inclined, the sea being supposed to be smooth and still. A “steady ship” is one which when exposed to the action of waves keeps nearly upright. Crank ships are usually the steadiest ships. Changes in the height of the point cf intersection M (fig. 4) above the centre of gravity indicate corresponding changes in the stiffness of a ship. Speaking generally, the stiffness of the ship may be considered to vary with the height of M above G. The line BM does not cut GM in the same point at considerable inclinations as it does at a very small inclination. The point of intersection at the smallest conceivable inclination receives a definite name. It is known as the metacentre, and the distance GM is in this condition called the metacentric height. See Hydromechanics.

The following table contains particulars of the metacentric heights of different kinds of vessels of war, and the corresponding time of an oscillation in still water :— [table]

Generally speaking, decrease in metacentric height is accompanied by a lengthening of the period of an oscillation. The ship swings more slowly as she loses stiffness.

There is no sensible difference in the time occupied by a ship in a swing or roll from side to side, whether she rolls through only three or four degrees on either side of the upright or twelve or fifteen degrees. For larger angles there would be small differences.

The tables which have been given show some remarkable changes in the stability conditions in ships of war within recent years. Sailing ships were formerly made with so little deviation from existing types that it was not found to be necessary to ascertain their exact measure of stability or to lay down rules for regulating it. The position of the centre of gravity was modified by ballast, and as much as nine or ten per cent. of the displacement was allowed for this. Heavy rolling and great uneasiness of ship from excessive stability had often to be endured. In other cases crankness or inability to carry sail had to be accepted. When armoured ships were first introduced they had about the same metacentric height (6 feet) as is to be found in the earlier sailing frigates. The “Normandie” in the French navy and the “Prince Consort” in the English navy had from 6 to 7 feet, and they were exceedingly uneasy and deep-rolling ships. It was soon discovered that a reduction in metacentric height would cure this evil. The later ships in both navies were accordingly designed to have a metacentric height of about 3 feet. The “Magenta” had 3¼ feet and the " Hercules” 3 feet. This change altered the period during which the ship made a double oscillation, i.e., from starboard back to starboard, to 14 to 16 seconds instead of 10 to 11 seconds, as it had been in the “Normandie” and “Prince Consort.” The effect on the behaviour of the ship in a seaway was most remarkable. These ships with small metacentric height might be put into the trough of a sea, and as the waves crossed them they steadily rose and fell, hardly inclining their masts. The effect on gunnery practice was also valuable, but there is always a peril attending steadiness obtained by such means: vessels having small metacentric height require careful handling under sail or they may be overset and lost. There is another defect in this system, viz., that wounds in action will cause the ship to incline sooner and more considerably, and they become more dangerous than they would be in a stiffer ship. Bilge-keels and water-chambers are now employed in the English navy, together with, and as opposing influences to, much greater metacentric height.

These devices were introduced into the “Inflexible” in order to counteract the influence of a metacentric height of 8 feet which was designedly given to her. They have proved very effective, but there is another feature in this vessel which has tended to prevent uneasiness and heavy rolling. The time of an oscillation, or quickness of rolling, depends not only upon the metacentric height but upon the moment of inertia about a longitudinal axis. The time of an oscillation from starboard to starboard may be written thus :—

2T = 1∙1√K 2 ∕m, where T is the ship’s period in seconds for a single roll, m is the metacentric height, or height of metacentre above the centre of gravity in feet, and K is the radius of gyration in feet. The moment of inertia is increased by widening the ship, putting heavy armour on her sides, and placing the turrets and guns out towards the sides of the ship. It was seen that these features in the “Inflexible,” which were elements in her design, would favour her and tend to counteract the great metacentric height. The event has shown that, while a metacentric height of 6 feet in the “Normandie” gave 10 seconds to 11 seconds period, 8 feet in the “Inflexible” only gives 11 seconds as a period, corresponding with a radius of gyration of 28 feet. The feeling expressed that “in order to provide against the impossible contingency of the loss of stability by complete waterlogging of the ends we had made an intolerable ship” was not justified. The ship is now so stiff that when the ends are waterlogged the running in and out of all her guns on one side only inclines her 2 1/2 degrees, while in the " Monarch” when intact and light the same operation inclines the ship 5 degrees.

The resistance offered by the water to the rolling of the ship consists of three parts :—(1) that due to the rubbing of the water against the bottom of the ship as she rolls; (2) that due to the flat surfaces which are carried through the water, such as outside keels and deadwood; (3) the creation of waves by the rolling ship to replace those which move away from the ship. The creation of these surface waves expends energy and checks the motion of the ship which makes the creative effort.

Mr White, giving briefly the results of some of the experiments of Mr Fronde made for the Admiralty, says :—

“Experiments have been made by Mr Froude to show how rapidly the rate of extinction may be increased by deepening bilge-keels. A model of the 'Devastation' was used for this purpose, and fitted with bilge-keels, which, on the full-sized ships, would represent the various depths given in the following table. The model was one thirty-sixth of the full size of the ship, and was weighted so as to float at the proper water-line, to have its centre of gravity in the same relative position as that of the ship, and to oscillate in a period proportional to the period of the ship. In smooth water it was heeled to an angle of 8 1/2 degrees, and was then set free, and allowed to oscillate until it came practically to rest, the number of oscillations and their period being observed. The following results were obtained :— [table]

“Not content with obtaining the aggregate value of the resistances for ships, Mr Froude has separated them into their component parts, assigning values to frictional and keel resistances as well as to surface disturbance. In doing so, he has been led to the conclusion that surface disturbance is by far the most important part of the resistance offered to rolling, as the following figures given by him for a few ships will show :— [table]

“Frictional and bilge-keel resistances in this table have been obtained by calculation from the drawings of the ship, Mr Froude making use of data as to coefficients for friction and for head resistance which he had previously obtained by independent experiments, and which may therefore be regarded as leading to thoroughly trustworthy results. It will be noticed that in no case does the sum of the frictional and keel resistances much exceed one-fourth of the total resistance, while it is much less than onefourth in other cases. The consequence is that surface disturbance must be credited with the contribution of three-fourths or thereabouts of the total resistance, a result which could scarcely have been predicted. Waves are constantly being created as the vessel rolls, and as constantly moving away, and the mechanical work done in this way reacts in a reduction of the amplitude of successive oscillations. Very low waves, so low as to be almost imperceptible, owing to their great length in proportion to their height, would suffice to account even for this large proportionate effect. For example, Mr Froude estimates that a wave 320 feet long and only 1 1/4 inches in height would fully account for all the work credited to surface disturbance in the fourth case of the preceding table.

“Another important deduction from the figures in the table is the large proportionate effect of 'keel' resistance as compared with frictional resistance, thus establishing the advantages of deep bilge-keels. Ships of the Royal Navy recently constructed have been furnished with much deeper bilge-keels than were formerly in use, but a limit to the depths that can be fitted is often reached, because of the necessity for compliance with certain conditions and extreme dimensions, in order that the vessels may be able to enter existing docks.”

In the Royal Navy advantage has also been taken of the power of loose water within a ship to quell motion. It was first employed in the “Inflexible” in a part of the ship lying above the bombproof deck, and at the level of the water-line. Its use resulted from a discussion, when the “Inflexible” was designed, of the probable effect of water entering this region of the ship through shot holes. The matter has since been thoroughly established by experiment, and affords a new and valuable means of preventing heavy rolling in ships having large initial stability. There is now no hesitation in giving a metacentric height of 6 feet, and obtaining all the security against upsetting which this ensures, because it is felt that the violent rolling formerly inseparable from stiffness can be prevented. The investigation into this matter has been conducted by Mr Philip Watts, Mr R. E. Froude, and Mr W. E. Smith, acting for the Admiralty.

The accompanying memorandum, prepared by the present writer in 1884, gives the general results :—

‘In investigating the phenomena attending the use of water as a means of quelling the motion of ships, Mr Froude has not only taken advantage of the experiments made in the ‘Edinburgh’ by running men across the decks, but he has also studied similar phenomena in a model water-chamber, mounted, rot on a model of a ship, but on a large pendulum weighted to the required ‘period,’ the relative level of the model chamber and the axis of rotation being made to correspond approximately to scale with that in the ship.

"The conclusions, stated in the form of a comparison between the quelling effects of bilge-keels and of moving water, are as follows:— (1) There is a certain depth of water in the chamber which gives the maximum effect; this is dependent upon the width of the chamber and the period of the ship. (2) With this depth of water the growth of resistance to rolling commences almost at zero of angle, whereas with either a greater or less depth there is practically no resistance at all due to the water up to a certain angle, which angle increases with increase of departure from the proper depth. (3) At larger angles of roll the disadvantage of departure from the proper depth of water is not marked. (4) The resistance of water in a chamber does not increase at all uniformly with increase of angle of roll, but increases rapidly at first and at the larger angles becomes more nearly constant for all angles. (5) The best quantity of water for the original chamber in the ‘Edinburgh’ was 43 tons; the best for the chamber enlarged by removal of cork walls was 79 tons; and the best for the chamber extending to the sides of the ship would be 100 tons. The first-named extension improved the resistance at 10° by 21 per cent., and the further extension by another 22 per cent.

“As compared with bilge-keels the matter is stated as follows while 2 feet addition to the breadth of the bilge-keels adds in round numbers two-thirds to the existing extinguishing power of hull and bilge-keels on the 'Edinburgh' at all angles of rolling, the fully extended water-chamber adds at 3° of roll about six times, and at 5° about three times that power; at 12° the chamber adds no more than 2 feet of bilge-keel, while at 18° it only adds half as much. It is therefore evident that, while both are valuable, the water-chamber is for most kinds of service much the more valuable of the two.

“Explaining the cause of the phenomena, Mr Froude says :—

“ ‘The extinguishing or quelling effect of water depends, of course, cae teris paribus, upon the value of the moment represented by the transference of water from side to side, i.e., with a given quantity of water, upon the distance moved by its centre of gravity. This distance increases with increase of angle of roll, and consequently the extinction similarly increases up to a certain point, where we appear to have approximately reached the maximum possible transference of water, and consequently the maximum extinction of which the quantity of water is capable with the dimensions of the chamber. This point occurs generally at a moderate angle, and above this angle the extinction becomes practically constant. But the extinguishing effect of the water of course largely depends also upon the timing of its motion from side to side,—the extinction being greatest when that motion takes place most nearly at the time of extreme angle of ship, i.e., in such a manner ns that the water may be as much as possible running downhill when it is moving across, and as much as possible upon the rising side of the ship when it is stationary. If, on the other hand, the motion of the water across were to take place when the ship is quite upright, the extinction would be nil. It is therefore conceivable that for the same total degree of motion or transference of water we may have a very different degree of extinction, according to the timing of that motion. In the motion of the water energy is necessarily wasted, and it is clear that, if we are dealing with a permanent condition of things, i.e., if the ship is being steadily maintained at a constant angle of roll, this waste of energy in each run of water from side to side must be exactly equal to the energy taken out of the ship in each swing by the extinction. The motion of the water may be and generally is of a type very wasteful of energy, the water either rushing across in a mass, and consuming its energy by breaking with great violence against the opposite side, as it does at the larger angles of rolling, or, at more moderate angles, running across in a breaking wave or bore, which consumes its energy as it goes in its own internal resistance; and under these circumstances the timing of the motion appears invariably to approach pretty nearly to that giving the maximum extinction for the degree of motion. But the motion of the water sometimes takes the form of a mere alternating slope of surface, or tidal swing from side to side, and here there is very little waste of energy, the energy of motion of the flow of water in one direction being converted into potential energy in the shape of rise of water at the side, and then given out again to the water flowing back to the other side, and so on. The waste of energy in this form of motion being almost nil, the timing is almost exactly that appropriate to no extinction, the water being in the middle of its passage across and the surface being level when the ship is upright.’

“The value of the chamber of course increases as its length in the direction of the keel of the ship increases. The actual size of the chamber we adopted appears to give valuable results, although its extent was necessarily limited.” [table]

Iu some lectures recently delivered Mr Smith, assistant constructor of the navy, illustrated the use of water in quelling motion by models as shown below.

“The models represented the midship sections of the ‘ Admiral ’ class, and were both of the same weight and size. Each model was mounted on trunuions, marked T, and both oscillated freely on these trunnions in exactly the same time. The models were placed one behind the other, so that the parallelism of the masts was evident to the audience. The model in fig. 7 was provided with a glass tube into which varying quantities of water could be put. An amount of water representing 1/100 of the total weight of the model, f.e., 100 tons in a 10,000-ton ship, was now placed in the tube, the models were started from the same angle as before, and the model with the loose water, instead of keeping up exactly with the other, or rolling more violently, came almost instantaneously to rest.

“The tube was filled with varying quantities of water, and the effect was always to stop the model much sooner than the model with no weights free to move. The two models were always started from the same angle, so that their relative behaviour could be easily seen. When the tube was quite full there was practically no effect. The two models rolled almost together. The same effect resulted from the motion of a marble representing in weight 100 tons in a ship of 10,000 tons. The same reduction must always occur in a rolling ship if we have a loose weight of any kind whether the weight be water or a gun. If this reduction did not take place we should have something to explain which would be quite inexplicable. For suppose we have two ships alike in all respects as regards size, shape, weight, time of oscillation, &c., and situated on precisely the same seas, but one having all her weights properly secured, and the other with a weight capable of traversing the deck every time the ship rolls. If the two vessels were to roll to exactly the same extent we should have the sea not only rolling the ship with the loose weight to the same extent as the ship with all her weights fixed, but the sea would, in addition, be doing all the work involved in the traversing of the heavy weight across the deck, which is quite impossible under the circumstances of perfect similarity we have supposed. The sea can only do the same work on both. In the one case that work consists entirely in rolling the vessel, in the other it consists partly of rolling the ship and partly in dashing the weight about. The rolling in the latter must therefore inevitably be less than in the former case.”

Dynamical stability is the “work” done or energy expended in heeling the ship from the upright to any inclined position. The unit of “work” employed in measuring dynamical stability is a foot-ton. When the vessel is gradually inclined the forces inclining her must do work depending upon the amount of the statical stability at the successive instantaneous inclinations passed through, and these are given by the curve of stability already described. Dynamical stability is of value as a means of comparing the resistance of ships to upsetting under the action of suddenly’ applied forces, such as squalls of wind. Illustrating this Mr White says :—

“Roughly speaking, it may be said that a force of wind which, steadily and continuously applied, will heel a ship of ordinary’ form to a certain angle will, if it strikes her suddenly when she is upright, drive her over to about twice that inclination, or in some cases further still. A parallel case is that of a spiral spring; if a weight be suddenly brought to bear upon it, the extension will be about twice as great as that to which the same weight hanging steadily’ will stretch the spring. The explanation is simple. When the whole weight is suddenly brought to bear upon the spring, the resistance which the spring can offer at each instant, up to the time when its extension supplies a force equal to the weight, is always less than the weight; and this unbalanced force stores up work which carries the weight onwards, and about doubles the extension of the spring corresponding to that weight when at rest.”

Structure.

The changes which have come about in materials and modes of construction within the last 50 years have been most remarkable. The first steamer built expressly for regular voyages between Europe and America was not built until 1837. Dr Lardner stated at about this date: “We have as an extreme limit of a steamer’s practicable voyage, without receiving a relay of coals, a run of about 2000 miles.” The “Great Western,” built by Patterson of Bristol and engined by Maudslay of London under the superintendence of Sir I. K. Brunel, was the first such ship, and she was launched July 19, 1837. She was 212 feet long between the perpendiculars, 35 feet 4 inches broad, and had a displacement of 2300 tons. She was propelled by paddles. Iron vessels were built early in the present century for canal service, then for river service, and later for packet service on the coasts. In about the year 1838 iron vessels of small dimensions were built for ocean service. The largest iron vessel built up to 1841 was less than 200 feet long. In 1843 we get for the first time the ocean-going steamship in its present form, built of iron, " and propelled by’ the screw. This was the “Great Britain,” 286 feet long, projected and designed by Brunel. Time has abundantly justified these bold enterprises on the part of Brunel, which he had to carry’ through in the face of great opposition. He entered with equal boldness on another innovation in 1850, viz., the use of very large dimensions on the ground of economy of power. It was not until 1852 that he had the oppor tunity to put these views forward in a way to satisfy him. The different sizes of vessels discussed before the design was finally settled for the " Great Eastern” were as follows :— [table]

The dimensions eventually settled were—length, 680 feet; beam, 83 feet; mean draught, about 25 feet; screw engine, indicated horse-power, 4,000, and nominal horse-power, 1600; paddle, indicated horse-power, 2,600, and nominal horse-power, 1,000; to work with steam 15 lb to 25 lb; speed of screw, 45 to 55 revolutions; paddle, 10 to 12.

The “Great Eastern,” produced by the joint skill of Brunel and Scott Russell, remains in advance of present practice, although she has served as a model for the best of it. Her great size rendered it possible to give to her an amount of security’ against fatal injury to her hull which cannot be attained in smaller ships. It is a mistake to suppose that large ships are less secure than small ones. The large ship can receive without inconvenience a wound which would be fatal to a small one, and the possibilities of obtaining high speed increase with the size. Had a higher speed been aimed at in the “Great Eastern,” it might have altered the whole current of her history, and changed also the history of shipbuilding itself.

The question of bulkheads, on which Brunel insisted so much in this ship, is one which underlies all questions of construction. If the number of bulkheads in ships were increased as they ought to be, the numbers and sizes of the ribs or frames of the ship would be modified, and the system of construction generally would be changed, and become more like that of the “Great Eastern.” The question is therefore one which justifies some further consideration, so that it may’ be popularly understood.

Iron ships are commonly made with less than half their bulk out of water. If water enters such a ship, and the amount which enters does not exceed in bulk that portion of the bulk of the ship which is out of the water, and which will, when immersed, exclude the water, then the ship, if she does not turn over, will still float. If, however, the inflow cannot be stopped, but continues, the ship soon sinks.

Let us suppose the case of a ship 50 feet long, 10 feet wide, and 10 feet deep, divided into five equal parts by four watertight partitions, and floating in water with half its bulk immersed (fig. 8). Suppose now that a hole is made in the middle of this ship under the water, so that water can flow freely in, then the part of the ship which is shaded ceases to have floating power. The water in this shaded place is no longer displaced, but is admitted, and if the ship is to continue afloat, the other parts of the ship must displace water to the amount by which this shaded part has ceased to do so. As it is one-fifth of the whole immersed bulk which is lost, the remaining four compartments must sink, so as each to support one-fourth of the whole, instead of one-fifth, as before; i.e ., the draught of water, or immersion of the whole ship, will be increased, and the ship will, if she has stability enough to keep upright, finally’ float at rest again at this deeper immersion. The water will rise in the centre compartment to the level of the water outside, and will then cease to flow in. The additional immersion, will be only’ one and a quarter feet, but in an ordinary’ ship, divided into compartments of equal length, there would be a greater increase of immersion by the injury of a centre compartment, because the end compartments are narrow, and must sink deeper in order to bear their share of the burden imposed by the loss of the buoyancy of the centre division.

Or it may be other than a central compartment which is damaged, and in that case the ship tips, and finds a new floating line, with the end towards which the damaged division lies depressed more than the other end.

If it should happen that the divisional partitions, or bulkheads as they are called, rise only a few inches above the water level which the ship floats at when undamaged, then, on the occurrence of a bad leak filling one compartment, the tops of the bulkheads are brought, by the increased immersion of the ship, beneath the water-level, the water will rise through the hatches, or openings in the deck, in the damaged compartment, will flow over the entire deck, and the ship will be lost, either by the filling of other compartments by the water passing down into them, or by the capsizing of the ship. This latter event will generally’ happen, although only one compartment is full, if the sea has free access to the deck from end to end of the ship, and it becomes wholly immersed.

In 1866 the president of the Institution of Naval Architects said: “The circumstances of the sad event of the loss of the ‘London,’ accompanied as it was by the simultaneous loss of another ship of still larger size, and of a higher reputed character” (the “Amalia,”), “was, I think, an event so remarkable that I should be very sorry indeed if this annual meeting of this Institution were to pass by without some notice being taken of the extraordinary circumstances of the loss of that ship, and without some discussion upon what we suppose to be the causes of the loss, and the faults, if any, of the construction of those ships.” “The passengers who pass to and fro are not judges of the question; they can take no precaution for their own safety; it is to the skill and science of those who build these ships that the passenger trusts, and to the care which the legislature and the Government are bound to take of their fellow-subjects.”

Subsequently the council of the Institution arrived at the following conclusions and offered them as recommendations to shipbuilders and shipowners :—

“1. No general rule can be safely laid down for regulating the proportions of length and depth to the breadth of a ship, and a great variety of proportions of length and depth to breadth may he safely adopted, and the ship made sound and seaworthy, by judicious form, construction, and lading.

“2. The construction load-water-line of every ship, and her scale of displacement from light to load-water-line, should be appended to every design of a ship, showing the extreme draught to which she should be laden; and measures should be taken to ensure that this information be recorded on the ship’s papers. It is desirable also that along with a ship’s papers, in the possession of the captain, there should always be carried a scale of displacement, a sail draft, and a set of outline plans of the ship, comprising a longitudinal section, and at least four cross sections of the ship. On these plans should be marked the capacity, in tons of 40 cubic feet, of each compartment of the hold. The surplus buoyancy of each compartment up to the load-water-line, or its power to carry deadweight, should be given in tons deadweight. These papers should always accompany the ship’s register, and a copy of them should be lodged in the custom house of the port from which the ship hails.

“3. There is a minimum height of freeboard which cannot be safely reduced in sea-going ships of ordinary fitment; and it is desirable to fix this minimum height. Freeboard should be understood to be the vertical height of the upper surface of the upper deck (not spar-deck) at the side, amidships, above the load-water-line. The proportion of freeboard should increase with the length. One-eighth of the beam is a minimum freeboard for ordinary sea-going ships of not more than five breadths to the length, and 1/32 of the beam should further be added to the freeboard for each additional breadth in the length of the ship; this would give—

For a ship of 32 feet beam and 160 feet long, 4 feet freeboard;

For a length of 192 feet, 6 feet freeboard;

For a length of 224 feet, 6 feet freeboard;

For a length of 256 feet, 7 feet freeboard the beam remaining the same. But, as the addition of a spar-deck on long vessels may be considered an equivalent or substitute for the increased freeboard required for extra length, a complete spar-deck would leave the freeboard of these extra lengths at the original height of 4 feet.

“4. It is not considered desirable to offer any recommendations with regard to poops and forecastles. It must depend entirely upon the professional judgment of the designer of a ship, whether, looking to her proportions, form, and purpose, the additions of poop and forecastle are expedient and safe. In general, where poops and forecastles are adopted, they should be closed and seaworthy, but their weight may be inexpedient in long fine ships; and there are cases where a light top-gallant forecastle (i.e. , an open forecastle raised above the level of the upper deck) may be useful in keeping heavy seas out of the ship. In general, spar-decks in long ships are preferable to poop and forecastle, and no diminution of freeboard should be allowed for a poop or forecastle.

“5. It would add much to the strength and security of steamships if transverse and longitudinal bulkheads, coal bunkers, iron lower decks, and screw alley were all so connected with the hull of the ship and with each other as to form independent cellular compartments, watertight, and having all their communications with the decks and each other by watertight doors worked from the deck. In proportioning the compartments of a ship (and especially of ships devoted to passengers) it is very desirable so to arrange them that if any two adjacent compartments be filled, or placed in free communication with the sea, the remaining compartments will float the ship. It is considered that no iron passenger ship is well constructed unless her compartments be so proportioned that she would float safely were any one of them to fill with water, or be placed in free communication with the sea. Double bottoms are to be regarded as a great element, both of safety and strength, in the structure of a large iron ship.

“6. It is very desirable that sufficient ventilation should always be provided in passenger ships to admit of closing all side scuttles and battening down, or otherwise enclosing, all hatches in bad weather.

“7. In regard to hatchways and openings in the deck no limits can be set to their size; but it is desirable to carry the beams of the ship across them without interruption wherever practicable; they may also be made removable where required, being replaced on going to sea. All coamings over engine and boiler rooms in passenger ships should be as high as practicable, of iron, and riveted to the beams and carlings. Openings in the deck may be fitted with solid coverings, hinged in place so as to be readily closed.

“8. It being considered that all openings in the sides or ends of vessels are subject to accidents that endanger the safety of ships, it is desirable that the side and stern windows should, in addition to the glass lights, have hinged dead-lights, with a view to their being always in place, and that all cargo ports should be strongly secured by iron cross bars.

“9. It is believed that all openings from and communications with the sea from engine-room and pipes should be protected by conical, or Kingston, or sluice valves, and similar precautions should be taken for all openings through the bottom of the ship, where damage to pipes or ship would admit water into the holds.

“10. It is considered that all steam vessels, if of iron, should have a brassbarrelled hand-pump to every compartment except the forward and after ones (the former to have a sluice cock), or that, as a substitute for these pumps, there should be patent pumps having independent connexions to this extent. They should also have a donkey engine and pump capable of pumping from the bilge and from the sea, of feeding the boilers, and of throwing water on deck. All vessels should have one or more bilge-pumps, worked by the large engines, with bilge injection pipes if the engines have condensers. In large vessels the donkey engines should have a separate boiler high above the water-line, and also communication with the main boilers. All vessels should have a set of bilge pipes connecting every hold and the engine compartments with these pumps. As a security against fire there should be pumps on the upper deck, fitted as force pumps, and provided with a sufficient length of hose (with the necessary copper delivery jets) to reach either extremity of the vessel, and also provided with suction hose or pipes from the sea. The cocks by which the working of the pumps is regulated should be carefully arranged and marked, and great care should be taken that both cocks and pipes are accessible. A plan of the whole should accompany the ship’s papers, and the crew should be periodically exercised in their use.

“11. The stowage of a ship, whether done by contract or not, should be done under inspection of the captain of the ship, and should be conducted under his own orders only; and he alone should be held responsible for the good stowage of his ship. Ships are often very badly stowed, the weights being sometimes too low, thus causing them to roll with such rapid and violent motions as to carry away the spars, and otherwise endanger the safety of the ship, and at other times too high, thus making the ships crank, and liable to turn over. A ship may, however, generally, whatever her form, be so stowed as to avoid both dangers. As the character of the ship in these respects varies, so does the number of oscillations she would make per minute if she were set rolling in still water, by men running across her deck, or other means, and then allowed to come to rest; that is, if the ship be crank the number of oscillations per minute will be few, and if she be too stiff they will be numerous; but, under the same conditions of stowage, the number will always be very nearly the same, whatever the amount of the impulse to set her rolling may be. Although this peculiarity has long been known to scientific men, no such observations have been made in merchant ships as would justify any specific rule on the subject. It is, however, most desirable that information should be collected upon it, and that the attention of the owners and captains of vessels should be called to it.

“12. It is believed that the present rules of the Board of Trade regarding boats, life-boats, and their tackle are good in principle. The responsibility for keeping all boats in constant readiness and efficiency obviously rests on the captain, ami must fix on him the blame for all neglect and its consequences. Every open boat built of iron or steel should be fitted with sufficient watertight spaces to float her.

“13. The system of proportioning anchors and cables by Lloyd’s, and of proving under licence of the Board of Trade by Act of Parliament is so far satisfactory; but, as the proof-test alone cannot establish the excellence of the cable, the reputation of the makers must be relied upon.

“14. In order to provide for the rapid clearance of the upper deck from water which may break over the ship, flap-boards should be fitted to the lower part of the bulwarks, sufficient in number and in area to admit of the rapid escape of the water.

“15. Water-closets on decks below or near the water-line may be the means of gradually and imperceptibly flooding the ship, and endangering her safety, unless the pipes and valves are strong and are carefully fitted.”

It is in the directions indicated in these recommendations that the honesty and skilfulness of the modern builder of steam and sailing ships of war come into play, and some judgment may be formed by the general public of the character of the ship by inquiring into matters upon which the council thought it necessary to make such recommendations. The guarantee which the public have of the fitness of passenger ships for service, as a question of proper construction and state of efficiency, is the survey and certificates of the Board of Trade. The law runs thus :—

The owner of every steam vessel constructed or intended to carry passengers (except vessels which fall within the definition of foreign-going ships contained in the Mercantile Marine Act, 1850, and are employed in the conveyance of the royal public mails or despatches under contract with and under the superintendence of the lord high admiral or the commissioners for executing the office of lord high admiral) shall cause such steam vessel to be surveyed twice at least in every year, at the times hereinafter directed, by a shipwright surveyor and by an engineer surveyor appointed for the purposes of this Act by the lords of the said committee, such shipwright surveyor in the ease of an iron steam vessel being a person properly qualified to survey iron steam vessels, and shall obtain a declaration of the sufficiency and good condition of the hull of such steamer, and of the boats and other equipments thereof, required by this Act; and also, if the lords of the said committee so require, a statement of the number of passengers (whether deck passengers or other passengers) which such vessel is constructed to carry, under the hand of such shipwright surveyor, and a declaration of the sufficiency and good condition of the machinery of such steamer under the hand of such engineer surveyor; and in such declarations it shall be distinguished whether such vessel is in construction and equipments adapted for sea service as well as for river or lake service, or for river or lake service only; such declaration shall state the local limits within which such vessel is, in the judgment of the surveyor, adapted for plying; and in the case of seagoing vessels the declaration of one of the surveyors shall contain a statement that he is satisfied that the compasses have been properly examined and adjusted; and such owner shall transmit such declarations to the lords of the said committee within fourteen days after the dates thereof respectively.

As to the fifth recommendation of the council of the Institution of Naval Architects, it must be observed that there is at present no law relating to the subdivision of steamships. There was a clause (No. 300) in the Merchant Shipping Act of 1854, which was virtually a reproduction of clause 20 of the Steam Navigation Act of 1851, and which read as follows :—

“1. Every steamship built of iron of 100 tons or upwards, the building of which commenced after the 28th day of August 1846, and every steamship built of iron of less burden than 100 tons, the building of which commenced after the 7th August 1851 (except ships used solely as steam tugs), shall be divided by substantial transverse watertight partitions, so that the fore part of the ship shall be separated from the engine-room by one of such partitions, and so that the after part of such ship shall be separated from the engineroom by another of such partitions.

“2. Every steamship built of iron, the building of which commences after the passing of this Act, shall be divided by such partitions as aforesaid into not less than three equal parts, or as nearly so as circumstances permit.

“3. In such last-mentioned ships each such partition as aforesaid shall be of equal strength with the side plates of the ship with which it is in contact. “4. Every screw steamship built of iron, the building of which commences after the passing of this Act, shall, in addition to the above partitions, be fitted with a small watertight compartment inclosing the after extremity of the shaft.”

The above law was repealed by the Act dated 29th July 1862, and on the 28th August 1863 the Admiralty applied to the Board of Trade to know whether the Board of Trade officers were empowered under any circumstances to insist on iron vessels having watertight compartments when employed in conveyance of mails and passengers, observing that the Admiralty were still of opinion that the regulations in force prior to the Amendment Act of 1862 in respect of contract packets should not have been relaxed. They considered such vessels should have compartments so arranged that if any one of them became filled with water the loss of buoyancy thereby occasioned should not endanger the safety of the ships, as recommended by them in their communication of the 17th December 1860. To this the Board of Trade replied (3d September 1863) that their surveyors no longer had any power to require given watertight partitions to be fitted in passenger steamships—though they agreed with the Admiralty in thinking that steam vessels carrying passengers and mails should be provided with a sufficient number of watertight partitions,—and had no reason to suppose that the Admiralty would not insist on such partitions being fitted in all steamships employed in conveyance of mails. They further say that the enactments in the Act of 1854 were repealed, not because of any doubts as to the necessity of proper and sufficient watertight partitions, but because those enactments which required only two of such partitions for all sizes and classes of ships had become practically useless or mischievous. It was found that in large vessels more partitions than the Act required were necessary to secure the safety of the ship, and it was thought better to leave builders and designers unfettered in providing extra strength and security to meet the various forms, sizes, and descriptions of ships than to tie them down by general statutory regulations which could not be so framed as to meet the varying wants and circumstances of the shipbuilding trade.

In a return by the Board of Trade to the House of Commons, dated 11th August 1875, setting forth the instructions issued to their surveyors under the Merchant Shipping Acts, 1854 to 1873, clause 26 reads—

"Surveyors should not refuse to grant a declaration for a vessel solely on the ground that bulkheads are not fitted, that the ordinary bulkheads are not watertight, or that the bulkheads fitted are otherwise defective, unless they are of opinion that the want of, or the defective state of, the bulkheads renders the ship unseaworthy, in which case they are fully justified in refusing to grant a declaration. They should, in all cases in which they refuse to grant a declaration for a vessel in consequence of defects relative to bulkheads, forward to the Board of Trade a full statement of their reasons for thinking that those defects render the hull of the vessel unseaworthy. Collision watertight bulkheads, at least, must be fitted in all seagoing steamers. The surveyors are also to see that an after watertight compartment is fitted to cover the stern-tube of the screw-shaft, both in old and in new vessels.”

This regulation has been reissued in the latest instructions to Board of Trade surveyors, dated 1884. It thus comes about that the number of bulkheads forming watertight compartments, the number of doors in them, and how they are fastened, are made the subject of consideration by the Board of Trade at their inspections; but the fact is that the great majority of ocean-going steamers are not divided into watertight compartments in any efficient manner, and many losses in collision, grounding, and swamping are due to this. Although all steamships have some bulkheads, and some have many bulkheads, they are as a rule distributed in such a way, or are so stopped below the water-level, that for flotation purposes after perforation those lying between the foremost collision bulkhead and the after bulkhead through which the screw shaft passes are practically useless.

"With the exception of some four hundred ships, there are no iron steamships afloat which would continue to float were a hole made in the bottom plating anywhere abaft the collision bulkhead and outside the engine-room, or which would not founder were water admitted through breaches made by the sea in weak superstructures and deck openings. Of the four hundred ships referred to as having properly designed bulkheads two hundred are essentially cargo-carriers. They are generally built with five subdivisions, the machinery space being one. Iron sailing ships are without exception undivided into compartments. They have by law a collision bulkhead near the bow, and that is all. Between June 1881 and February 1883 there were about one hundred and twenty iron steamships lost, of speeds of nine to twelve knots, not one of which was well constructed according to the opinion of the council of the Institution of Naval Architects.

It may be said that wooden ships were not divided into watertight compartments, but it must be remembered that in a wooden ship there is far more local resistance to a blow either in collision or by grounding, and that a wooden ship takes a much longer time to settle down in the water and sink. Also, when wood was employed for passenger and trading ships speeds were much lower and traffic and risks of collision very much less.

The shipbuilding registries prescribe rules for the government of the builder who desires to have their certificate, and these rules have been so carefully framed and so honestly enforced that English-built ships are as a rule well and solidly constructed. The recent (8th June 1882) rule of the London Lloyd’s register as to the important subject of division into compartments is as follows, and it may be hoped that it will be effective

“Screw-propelled vessels, in addition to the engine-room bulkheads, to have a watertight bulkhead built at a reasonable distance from each end of the vessel. In steamers 280 feet long and above an additional bulkhead is to be fitted in the main hold, extending to the main or upper deck, about midway between the collision and engine-room bulkheads; and in steamers of 330 feet long and above an additional bulkhead is to be fitted in the after hold, extending to the same height.”

“The foremost or collision bulkhead in all cases to extend from the floor plates to the upper deck. . . . The engine-room bulkheads to extend from the floor plates to the upper deck in vessels with one, two, or three decks, and to the main deck in sparand awning-decked vessels. The aftermost bulkhead will be required to extend to the upper deck unless the arrangement of bulkheads be submitted to and approved by the committee. . . . In sailing vessels the foremost or collision bulkhead only will be required.”

It is not intended by the foregoing remarks, serious as they are, to blot the splendid record of shipbuilding achievement in Great Britain during the last twenty years. The shipowners, shipbuilders, marine engineers, Lloyd’s surveyors, and the Board of Trade have all shared in a development of shipping which, in amount and in general efficiency, is not only without parallel in the history of the world, but, as it still appears to us who have witnessed it, almost incredible. It still is to be regretted that expansion has been thought of and sought more ardently than greater security and efficiency. The men who have studied to improve their structural arrangements because of their love of true and good work, and with no prospect of recognition or reward, have been comparatively very few.

There is, perhaps, no structure exposed to a greater variety of strains than a ship, and none in which greater risks of life and property are incurred. A thorough practical knowledge of the disturbing forces in action either to injure or destroy the several combinations embraced in its structure is therefore most important. Some of these forces always act, whether the ship be at rest or in motion. She may be at rest floating in still water, and will be at rest if cast on shore; and, when there, she may be resting on her keel as a continuous bearing, with a support from a portion of her side, or she may be supported in the middle only, with both ends for a greater or less length of her body left wholly unsupported, or she may be resting on the ends with the middle unsupported, or under any other modification of these circumstances; and under all these the strains will vary in their direction and in their intensity.

If the ship be in motion the same disturbing forces may still be in action, with others in addition which are produced by a state of motion. When a ship is at rest in still water, although the upward pressure of the water upon its body is equal to the total weight of the ship, it does not necessarily follow that the weight of every portion of the vessel will be equal to the upward pressure of that portion of the water directly beneath it, and acting upon it; on the contrary, the shape of the body is such that their weights and pressures are very unequal.

If the vessel be supposed to be divided into a number of laminae of equal thickness, and all perpendicular to the vertical longitudinal section, it is evident that the after laminae comprised in the overhanging stern above water, and the fore laminae comprised in the projecting head also above water, cannot be supported by any upward pressure from the fluid, but their weight must be wholly sustained by their connexion with the supported parts of the ship. The laminae towards each extremity immediately contiguous to these can evidently derive only a very small portion of their support from the water, whilst towards the middle of the ship’s length a greater proportionate bulk is immersed, and the upward pressure of the water is increased.

A ship floating at rest under the view just taken of the relative displacement of different portions of the body, if the weights on board are not distributed so that the different laminae may be supported by the upward pressure beneath them as equally as possible, may be supposed to be in the position of a beam supported at two points in its length at some distance from the centre, and with an excess of weight at each extremity. At sea it would be exposed to the same strain; and if supported on two waves whose crests were so far apart that they left the centre and ends comparatively unsupported, the degree of this strain would be much increased. The more these two points of support approach each other, or if they come so near each other that the vessel may be looked upon as supported on one wave, or on one point only in the middle of her length, the greater will be the tensile strain on the upper portion, and the crushing strain on the lower portion of the fabric of the ship. A vessel whose weights and displacements are so disposed as to render her subject to a strain of this kind beyond what the strength of her upperworks will enable her to bear, will tend to assume a curved form.

The centre may curve upwards by the excess of the pressure beneath it, and the ends drop, producing what is called “hogging.” The main remedy for these evils is in the strength of the deck and upperworks, and their power to resist a tensile strain. There is seldom a want of sufficient strength in the lower parts of the vessel to resist the crushing or compressing force to which it is subjected. The decks of vessels should not, therefore, be too much cut up by broad hatchways; and care should be taken to preserve entire as many strokes of the deck as possible. The tensile strength of iron can be brought to bear most beneficially in this respect.

Though these are the strains to which a ship is most likely to be exposed, it by no means follows that there are no circumstances under which strains of the directly opposite tendency, when pitching, or otherwise, may be brought by recoil to act upon the parts. The weights themselves in the centre of the ship may be so great that they may have a tendency to give a hollow curvature to the form, and it is therefore equally necessary to guard against this evil. When this occurs, the vessel is technically said to be “sagged,” in distinction to the contrary or opposite change of form by being hogged. The weight of machinery in a wooden steam-vessel, or the weight or undue setting up of the main-mast, will sometimes produce sagging. The introduction of additional keelsons tended to lessen this evil, by giving great additional strength to the bottom, enabling it to resist extension, to which, under such circumstances, it became liable; and, as the strain upon the deck and upperworks becomes changed at the same time, they are then called upon to resist compression.

When the ship is on a wind, the lee-side is subjected to a series of shocks from the waves, the violence of which may be imagined from the effects they sometimes produce in destroying the bulwarks, tearing away the channels, &c. The lee-side is also subjected to an excess of hydrostatic pressure over that upon the weather side, resulting from the accumulation of the waves as they rise against the obstruction offered to their free passage. These forces tend in part to produce lateral curvature. When in this inclined position, the forces which tend to produce hogging when she is upright also contribute to produce this lateral curvature.

The strain from the tension of the rigging on the weather side when the ship is much inclined is so great as frequently to cause working in the topsides, and sometimes even to break the timbers on which the channels are placed. Additional strength ought therefore to be given to the sides of tho ship at this place; and, in order to keep them apart, the beams ought to be increased in strength in comparison with the beams at other parts of the ship.

The foregoing are the principal disturbing forces to which the fabric of a ship is subjected; and it must be borne in mind that some of these are in almost constant activity to destroy the connexion between the several parts. Whenever any motion or working is produced by their operation between two parts, which ought to be united in a fixed or firm manner, the evil will soon increase, because the disruption of the close connexion between these parts admits an increased momentum in their action on each other, and the destruction proceeds with an accelerated progression. This is soon followed by the admission of damp, and the unavoidable accumulation of dirt, and these then generate fermentation and decay. To make a ship strong, therefore, is at the same time to make her durable, both in reference to the wear and tear of service and the decay of materials. It is evident from the foregoing remarks that the disturbing influences which cause “hogging” are in constant operation from the moment of launching the snip. As this curvature can only take place by the compression of the materials composing the lower parts of the ship and the extension of those composing the upper parts, the importance of preparing these separate parts with an especial view to withstand the forces to which they are each to be subjected cannot be overrated by the practical builder.

In his Manual of Naval Architecture, Mr W. H. White gives illustrations of tho still-water strains upon two armoured ships in the British navy the “Minotaur” and the “Devastation.”

In these diagrams the curves B represent the distribution of the buoyancy. The ordinates of the curve are proportionate to the displacement of ad jacent transverse sections of the ships. The curves W represent the distribution of the weight of the ships and their lading.

The curves L represent the excesses and defects of buoyancy obtained from the two curves B and W and set off from a new base line. The excess of buoyancy above the line is exactly equal to the defect of buoyancy below it. The curves M indicate the bending moments. The ordinates of tho curve lying above the base are obtained by summing all the moments, whether upwards or downwards, about the point in the length of the ship where the ordinate is taken. It may happen, as in the case of the “Devastation,” that the moments will tend to cause hogging for a portion of the length and will then change their character, and at other portions of the length will tend to cause sagging. Where the curve M crosses the base line there is no strain of either hogging or sagging tending to bend the ship there. In the ‘ ‘ Minotaur” there is a hogging tendency throughout. The amount at the midship section is very great, being represented by the moment 4 5 feet × 10·690 tons. After Sir Edward Reed left the Admiralty he strongly expressed his fears that this strain was too considerable for safety in the “Minotaur” and “Agincourt.”

Designing.

The principal plans of a ship are the “sheer” plan, giving in outline the longitudinal elevation of the ship; the “body” plan, giving the shape of the vertical transverse sections; and the “half-breadth” plan, giving the projections of transverse longitudinal sections. In addition to these the builder is furnished by the designer with elevations, plans, and sections of the interior parts of the ship, and of the framing and plating or planking.

The thicknesses or weights of all the component parts are specified in a detailed specification, in order that the ship when completed may have the precise weight and position of centre of gravity contemplated by the designer. In the case of ships built for the British navy all the building materials are carefully weighed by an agent of the designer before they are put into place by the builder. As each section of the work is completed, the weight is compared with the designer’s estimate in the designing office. As soon as the incomplete hull is floated the actual displacement is measured, and compared with the weights recorded as having gone into the ship. It is also the practice in the Royal Navy to calculate the position of the centre of gravity of the incomplete hull, and its draught of water before it is floated, in order to avoid all risk of upsetting from deficiency in stability at that stage of construction. The ship is usually found to float in precise accordance with the estimate. When completed ships float at a deeper draught than was intended, or are found to be more or less stable than was wished, this is nearly always due to additions and alterations made after the completion of the design. Where the designer is at liberty to complete the ship in accordance with the original intention there ought to be precise correspondence between the design and the ship.

In designing a ship of novel type the designer has to pass all the building details through his mind and assign them their just weights and proportions and positions. Every plate and angle bar and plank, every bar and rod and casting and forging, and every article of equipment has to be conceived in detail and its effect estimated.

Building.

The term “laying off” is applied to the operation of transferring to the mould loft floor those designs and general proportions of a ship which have been drawn on paper, and from which all the preliminary calculations have been made and the form decided. The lines of the ship, and exact representations of many of the parts of which it is to be composed, are to be delineated there to their full size, or the actual or real dimensions, in order that moulds or skeleton outlines may be made from them for the guidance of the workmen.

A ship is generally spoken of as divided into fore and after bodies, and these combined constitute the whole of the ship; they are supposed to be separated by an imaginary athwartship section at the widest part of the ship, called the midship section or dead-flat. The midship body is a term applied to an indefinite length of the middle part of a ship longitudinally, including a portion of tho fore-body and of the after-body. It is not necessarily parallel or of the same form for its whole length.

Those portions of a wooden ship which are termed the square and cant bodies may be considered as subdivisions of the fore-bodies and after-bodies. There is a square fore-body and a square afterbody towards the middle of the ship, and a cant fore-body and a cant after-body at the two ends. In the square body the sides of the frames are square to the line of the keel, and are athwartship vertical planes. In the cant bodies the sides of the frames are not square to the line of the keel, but are inclined aft in the fore-body and forward in the after-body. The reason for the frames in these portions of a wooden ship being canted is that, in these parts of the ship, the timber would be too much cut away on account of the fineness of the angle formed between an athwartship plane and the outline or water-line of the ship. The timber is therefore turned partially round till the outside face coincides nearly with the desired outline, and it is by this movement that the side of a frame in the cant fore-body is made to point aft, and in the cant after-body to point forward.

In wooden ships the term “timbers” is sometimes applied to the frames only, but more generally to all large pieces of timber used in the construction. Timbers, when combined together to form an athwartship outline of the body of a ship, are technically called frames, and sometimes ribs.

The keel, in the United Kingdom at least, is generally made of elm, on account of its toughness, and from its not being liable to split if the ship should take the ground, though pierced in all directions by the numerous fastenings passing through it. It is generally composed of as long pieces as can be obtained, united to each other by horizontal scarphs. The rabbet of the keel is an angular recess cut into the side to receive the edge of the planks on each side of it. The keel is connected forward to the stem by a scarph, sometimes called the boxing scarph, and aft to the sternpost by mortice and tenon. The apron is fayed or fitted to the after-side of the stem, and is intended to give shift to its scarphs, the lower end scarphs to the deadwood. The keelson is an internal line of timbers fayed upon the inside of the floors directly over the keel, the floors being thus confined between it and the keel. Its use is to secure the frames and to give shift to the scarphs of the keel, and thus give strength to the ship to resist extension lengthways, and to prevent her hogging or sagging. The foremost end of the keelson scarphs to the stemson, which is intended to give shift to the scarphs connecting the stem and keel. The frames or ribs are composed of the strongest and most durable timber obtainable.

The floors in the Government service were carried across the keel principal action of these forces is to alter the vertical angle made by the beam and the ship’s side—that is, to raise or depress the beam, and so alter the angle between it and the side of the ship above or below it. On the lee-side the weight of the weather side of the ship and all connected with it, and of the decks and everything upon them, as well as the upward pressure of the water, all tend to diminish the angle made by the beam and the ship’s side below it, and consequently increase the angle made between them above it. The contrary effect is produced on the weather side, where the tendency is to close the angle above the beam and open that below it. If the beam, when subjected to these strains, be considered as a lever, it will be evident that the fastenings to prevent its rising ought to be as far from the side as is consistent with the convenience or accommodation of the ship, and that, while the support should also be extended inwards, the fastening to keep down the beam-end should be as close to the end of the beam, and consequently to the ship’s side, as it can be placed.

The plank, or skin, or sheathing of a ship, both external and internal, is of various thicknesses. A strake of planking is a range of planks abutting against each other, and generally extending the whole length of the ship. A thick strake, or a combination of several thick strakes, is worked wherever it is supposed that the frame requires particular support—for instance, internally over the heads and heels of the timbers, both externally and internally in men-of-war vessels between the ranges of ports, and internally to support the connexion of the beams with the sides and at the same time form a longitudinal tie. The upper strakes of plank, or assemblages of external planks, are called the sheer-strakes. The strakes between the several ranges of ports, begin-

with a short and long arm on either side alternately, so as to break joint, and between the frames the space was filled in solid.

Longitudinal pieces of timber are worked round the interior of a ship for the purpose of receiving the ends of the beams of the several decks; they are called shelves, and are of the greatest importance, not only for this purpose, but also as longitudinal ties and struts.

The beams of a ship prevent the sides from collapsing, and at the same time carry the decks. The beams are spaced, and their scantling settled upon, according to the strength required to be given to the decks, and to suit the positions of the masts and hatchways, and other arrangements connected with the economy of the ship. All beams have a curve upwards towards the middle of the ship, called the round-up. This is for the purpose of strengt h, and for the convenience of the run of the water to the scuppers. Wooden beams are single piece, two, three, or four piece beams according to the number of pieces of timber of which they are composed. The several pieces are scarphed together, and dowelled and bolted, the scarphs being always vertical.

The connexion of the ends of the beams to the sides of the ship has been made in various ways. The points to be considered, with reference to this connexion, are—that the beam is required to act as a shore or strut, to prevent the sides of the ship from collapsing, and also as a tie to prevent their falling apart, that the beam shall not rise from its seat, and that it shall not work in a fore-and-aft direction.

That the beam may be an effective shore, nothing more is necessary than that the abutment of the end against the ship’s side may be perfect. In order that it may act as a tie between the two sides, it is generally dowelled to the upper surface of the shelf on which it rests; and the under surface of the waterway plank which lies upon it is sometimes dowelled into it. These dowels, therefore, connect it with the shelf and the waterway, and through this means it is thus connected with the sides of the ship.

From the short outline previously given of the disturbing forces acting on a ship it will be seen that the strain on the ends of the beams to destroy their connexion with the side and loosen the fastenings must be very great when the ship is under sail, either on a wind or before it—that is, either inclined or rolling. The ning from under the upper-deck ports of a three-decked ship in the British navy, were called the channel wale, the middle wale, and the main wale. The strake immediately above the main wale was called the black strake. The strakes below the main wale diminished from the thickness of the main wale to the thickness of the plank of the bottom, and were therefore called the diminishing strakes. The lowest strake of the plank of the bottom, the edge of which fits into the rabbet of the keel, is called the garboard strake.

Plank is either worked in parallel strakes, when it is called “straight-edged,” or in combination of two strakes, so that alternate seams are parallel. There are two methods of working these combinations, one of which is called “anchor stock,” and the other “top and butt.” The difference will be best shown by fig. 13. The difference in the intention is that in the method of working two strakes anchor-stock fashion, the narrowest part of one strake always occurs opposite to the widest part of the other strake, and consequently the least possible sudden interruption of longitudinal fibre, arising from the abutment, is obtained. This description, therefore, of planking is used where strength is especially desirable. In top and butt strakes the intention is, by having a wide end and a narrow end in each plank, to approximate to the growth of the tree, and to diminish the difficulty of procuring the plank. When the planking is looked upon as a longitudinal tie, the advantage of these edges being, as it were, imbedded into each other is apparent, all elongation by one edge sliding upon the other being thus prevented. The shift of plank is the manner of arranging the butts of tho several strakes. In the ships of the British navy the butts were not allowed to occur in the same vertical line, or on the same timber, without the intervention of three whole strakes between them. Of the internal planking the lowest strake, or combination of strokes, in the hold, is called the limber-strake. A limber is a passage for water, of which there is one throughout the length of the ship, on each side of the keelson, in order that any leakage may find its way to the pumps.

The whole of the plank in the hold is called the ceiling. Those strokes which come over the heads and heels of the timbers are worked thicker than the general thickness of the ceiling, and are distinguished as the thick strokes over the several heads. The strokes under the ends of the beams of the different decks in a man-of-war, and down to the ports of the deck below, if there were any ports, were called the clamps of the particular decks to the beams of which they are the support—as the gun-deck clamps, the middle-deck clamps, &c. The strokes which work up to the sills of the ports of the several decks were called the spirketting of those decks—as gun-deck spirketting, upper-deck spirketting, &c.

The fastening of the plank is either “single,” by which is meant one fastening only in each stroke as it passes each timber or frame; or it may be “double,” that is, with two fastenings into each frame which it crosses; or, again, the fastenings may be “double and single,” meaning that the fastenings are double and single alternately in the frames as they cross them. The fastenings of planks consist generally either of nails or treenails, excepting at the butts, which are secured by bolts. Several other bolts ought to be driven in each shift of plank as additional security. Bolts which are required to pass through the timbers as securities to the shelf, waterway, knees, &c., should be taken advantage of to supply the place of the regular fastening of the plank, not only for the sake of economy, but also for the sake of avoiding unnecessarily wounding the timbers.

The decks of a wooden ship must not be considered merely as platforms, but must be regarded as performing an important part towards the general strength of the whole fabric. They are generally laid in a longitudinal direction only, and are then useful as a tie to resist extension, or as a strut to resist compression. The outer strokes of decks at the sides of the ship are generally of hard wood, and of greater thickness than the deck itself; they are called the waterway planks, and are sometimes dowelled to the upper surface of each beam. Their rigidity and strength is of great importance, and great attention should be paid to them, and care taken that their scarphs are well secured by through-bolts, and that there is a proper shift between their scarphs and the scarphs of the shelf.

When the decks are considered as a tie, the importance of keeping as many strokes as possible entire for the whole length of the ship must be evident; and a continuous stroke of iron or steel plates beneath the decks is of great value in this respect. The straighter the deck, or the less the sheer or upward curvature at the ends that may be given to it, the less liable will it be to any alteration of length, and the stronger will it be. The ends of the different planks forming one stroke were made to butt on one beam, and, as the fastenings are driven close to the ends, they did not possess much strength to resist being torn out. The shifts of the butts, therefore, of the different strokes required great attention, because the transference of the longitudinal strength of the deck from one plank to another was thus made by means of the fastenings to the beams, the strokes not being united to each other sideways. The introduction of iron decks or partial decks under the wood has modified this.

These fastenings have also to withstand the strain during the process of caulking, which has a tendency to force the planks sideways from the seam; and, as the edges of planks of hard wood will be less crushed or compressed than those of soft wood when acted on by the caulking-iron, the strain to open the seam between them to receive the caulking will be greater than with planks of softer wood, and will require more secure fastenings to resist it. It may also be remarked that the quantity of fastenings should increase with the thickness of the plank which is to be secured, for the set of the oakum in caulking will have the greater mechanical effect the thicker the edge.

When the planks are fastened, the seams or the intervals between the edges of the strokes are filled with oakum, and this is beaten in or caulked with such care and force that the oakum, while undisturbed, is almost as hard as the plank itself. If the openings of the scam were of equal widths throughout their depth between the planks, it would be impossible to make the caulking sufficiently compact to resist the water. At the bottom edges of the seams the planks should be in contact throughout their length, and from this contact they should gradually open upwards, so that, at the outer edge of a plank 10 inches thick, the space should be about 10/16 of an inch, that is, about 1/16 of an inch open for every inch of thickness. It will hence be seen that, if the edges of the planks are so prepared that when laid they fit closely for their whole thickness, the force required to compress the outer edge by driving the caulking-iron into the seams, to open them sufficiently, must be very great, and the fastenings of the planks must be such as to be able to resist it. Bad caulking is very injurious in every way, as leading to leakage and to the rotting of the planks themselves at their edges.

Ships are generally built on blocks which are laid at a declivity of about 5/8 inch to a foot. This is for the facility of launching them. The inclined plane or sliding plank on which they are launched has rather more inclination, or about ⅝ inch to the foot for large ships, and a slight increase for smaller vessels. This inclination will, however, in some measure, depend upon the depth of water into which the ship is to be launched.

While a ship is in progress of being built her weight is partly supported by lier keel on the blocks and partly by shores. In order to launch her the weight must be taken off these supports and transferred to a movable base; and a platform must be erected for the movable base to slide on. This platform must not only be laid at the necessary inclination, but must be of sufficient height to enable the ship to be water-borne and to preserve her from striking the ground when she arrives at the end of the ways. For this purpose an inclined plane a, a (fig. 14), purposely left unplaned to diminish the adhesion, is laid on each side the keel, and at about one-sixth the breadth of the vessel distant from it, and firmly secured on blocks fastened in the slipway. This inclined plane is called the sliding-plank. A long timber, called a bilge way b, b, with a smooth under-surface, is laid upon this plane; and upon this timber, as a base, a temporary frame-work of shores c , c , called “poppets,” is erected to reach from the bilge-way to the ship. The upper part of this frame-work abuts against a plank d, temporarily fastened to the bottom of the ship, and firmly cleated by cleats e , e, also temporarily secured to the bottom. When it is all in place, and the sliding-plank and under side of the bilgeway finally greased with tallow, soft soap, and oil, the whole framing is set close up to the bottom, and down on the sliding plank, by wedges f, f, called slivers or slices, by which means the ship’s weight is brought upon the “launch” or cradle.

When the launch is thus fitted, the ship may be said to have three keels, two of which are temporary, and are secured under her bilge. In consequcnce of this width of support, all the shores may be safely taken away. This being done, the blocks on which the ship was built, excepting a few, according to the size of the ship, under the foremost end of the keel, arc gradually taken from under her as the tide rises, and her weight is then transferred to the two temporary keels, or the launch, the bottom of which launch is formed by the bilgeways, resting on the well-greased inclined planes. The only preventive now to the launching of the ship is a short shore, called a dog-shore on each side, with its heel firmly cleated on the immovable platform or sliding-plank, and its head abutting against a cleat secured to the bilgeway, or base of the movable part of the launch. Consequently, when this shore is removed, the ship is free to move, and her weight forces her down the inclined plane to the water. To prevent her running out of her straight course, two ribands are secured on the sliding-plank, and strongly shored. Should the ship not move when the dog-shore is knocked down, the blocks remaining under the fore part of her keel must bo consecutively removed, until her weight overcomes the adhesion, or until the action of a screw against lier fore-foot forces her off.

A different mode of launching is sometimes practised in British merchant-yards, and has been long in use in the French dockyards, allowing the keel to take the entire weight of the vessel. The two pieces a, a, which arc shown in fig. 15 as being secured to the ship’s bottom, are the only pieces which need be prepared according to this system for each ship, the whole of the remainder being available for every launch. A space of about half an inch is left between them and the balk timber placed beneath them, as it is not intended that the ship should bear on these balk timbers in launching, but merely be supported by them in the event of her heeling over. The ship, therefore, is launched wholly on the sliding-plank c , fitted under the keel.

If a ship is coppered before launching, so that putting her into a dry-dock for that purpose becomes unnecessary, it is then desirable that she should be launched without any cleats attached to her bottom. The two sides of the cradle are prevented from being forced apart when the weight of the ship is brought upon them by chains passing under the keel. Each portion of frame-work composing the launch has two of these chains attached to it, and brought under the keel to a bolt which passes slackly through one of the poppets, and is secured by a long forelock, with an iron handle, reaching above the water-line, so that when the ship is afloat it may be drawn out of the bolt. The chain then draws the bolt, and in falling trips the cradle from under the bottom. There should be at least two chains on each side secured to the fore-poppets, two on each side secured to the after-poppets, and two on each side to the stopping-up, and this only for the launch of a small ship; in larger ships the number will necessarily be increased according to the weight of the vessel and the tendency that she may have, according to her form, to separate the bilgeways. This tendency on the part of a sharp ship by a rising floor, or by her wedge-shaped form in the fore and after bodies, is great, but there is not much probability of a ship heeling over to one side or the other.

The importance of the work of the designer cannot be too highly estimated. Unfortunately there is, as has been said, “slop work” in designing as well as in putting the structure together. There is often an absence of any attempt at precautions where multiplied accidents have shown them to be necessary, as well as inconceivable carelessness in the details rendering provisions for security, where they exist in principle, useless in practice.

In the Report of the Royal Commission on Unseaworthy Ships, dated September 22, 1873, we read as follows :—“Competent witnesses state that many merchant ships are built with bad iron, that they are ill put together, and sent to sea in a defective condition. It is also said that they are frequently lengthened without additional strength, and are consequently weak ships. The number of iron steamers which have been lost in the last few years, many of them having been surveyed and classed under the London or Liverpool registers, raises a question whether the regulations of these registers are sufficiently stringent to insure good shipbuilding. The directors of the Bureau Veritas have deemed it necessary to revise the rules of their register, and to increase the scantling. In the race of competition among shipbuilders it is probable that inferior materials and bad workmanship are admitted into ships.”

The Commissioners on Unseaworthy Ships, referring to the proposal that the Board of Trade should superintend the construction, the periodical inspection, the repair, and the loading of all British merchant ships, said: “We consider it to be a question worthy of serious consideration, whether, in the case of passenger ships, the certificate of the Board of Trade, so far as regards specific approval, should not be expressly confined to the number of passengers to be allowed and to the accommodation for their health, comfort, and general security,—all questions of unseaworthiness of hull, machinery, and equipment being left to the owners, subject only to a general power of interference in case of danger sufficiently apparent to justify special intervention.”

Where ships have to meet the stress of battle as well as that of the sea faithfulness of work is even more imperative. It is not only necessary to have perfect work, but there must also be multiplied safeguards and provisions against damage by shot, shell, ram, and torpedo as well as against the enemies which are common to all ships. In the article Navy the peculiarities of the ship of war are described. Regarding them here simply as ships, they may be said to be distinguished neither by size nor speed. They have been far outstripped in size, the longest English ship of war built within the last twenty years being only 325 feet in length, while there are Atlantic passenger ships 200 feet longer. They have also been outstripped in speed. The highest speed ever attained in a vessel of war is that of the “Iris” and “Mercury”; and as they are only 300 feet long it is easier in vessels of greater length to get higher speeds with less engine power, and easy also to maintain it in a seaway both as a question of form and power, and also as a matter of coal endurance. The following table gives the relative dimensions of large 14-knot ships :— [table]

ft. in.

435 7 / 41 8 = 10·45

The differences between the amount and complexity of fitting in the ship of war and the merchant ship are represented by the greatly increased cost per ton weight of hull. It must, however, be premised that the war ship has the weight of hull kept down to a very low standard to enable her to carry her offensive and defensive equipment,—far lower than is usual in the merchant ship. The first-class merchant ship costs £28 per ton weight of hull and about £13 per indicated horse-power for the engines. The ship of war built by the same builders under contract with the Government costs from £60 to £65 per ton weight of hull for unarmoured ships, and from £70 to £75 or more for armoured ships. In the case of an unarmoured vessel, having a protecting deck over machinery and magazines, recently ordered, the prices were as follows :—

General average £60 10 0 per ton weight of hull.

Average of three London firms. 66 0 0 ,, ,,

Accepted tender 57 6 0 ,, ,,

The engines for the same vessel were :—

General average £15 8 0 per I.H. P.

Average of three London firms 17 5 0 ,,

Accepted tender 11 8 0 ,,

In the case of a larger armoured ship the rates were :—

Average price per ton weight of hull £81 2 0

Accepted tender 71 5 0

Average price per I. H. P. of engines 11 1 0

Accepted tender 10 7 0 [table]

First-

Class

Passenger

Steamers.

Cargo

Steamers.

Armoured

Battle

Ships

(Barbette).

Protected

13-Knot Ships. Unarmoured, Masted, and

Sheathed.

Protected

10 to 12 Knots. Unarmoured,

Masted, and

Sheathed.

Torpedo

Boats,

19 to 20 Knots.

The use of heavy ordnance in recent times as the sole weapon for naval warfare brought about a marked distinction between the merchant vessel and the war ship, which had not previously existed. The revival of the ram and the adoption of the torpedo tend to abolish this distinction and to bring about an approximation again.

It is difficult to say what, in the very near future, will be the distinguishing characteristics of the ship of war. They will not be speed or size or coal endurance, or the power of striking with the ram, the torpedo, or the gun. It will be quite easy to arm merchant ships with these weapons, and some of these ships already outstrip the war vessel in the important advantages of size and fleetness and carrying power. It is apparently in protective advantages that the essential difference will lie.

The merchant ship is badly provided against fatal damage by collision, or by a blow delivered in any manner by which water is admitted into the ship. The propelling machinery of these ships and their steering apparatus are also dangerously exposed to artillery fire. Excepting torpedo boats, the ship of war of any size has its propelling machinery either under water or under cover of armour, and in a great number of cases there is either protection for the steering apparatus or there arc two propellers. The approximation towards war-ship arrangements which is needed in the merchant ship is the adoption of more than one screw and of greater breadth of ship, so that defences round machinery may be created in time of war. Both these changes in merchant-ship practice are demanded also by mercantile interests. The increase in breadth amidships would greatly reduce the risk of foundering in collisions and give more spacious accommodation amidships. Such increase when accompanied by fine ends is also favourable to speed.

The use of two screws is economical of power, and is a much-needed security against the evil results of an accident to an engine, a shaft, or a propeller. The time will doubtless come when a single propeller in a large passenger ship will be regarded as an unpardonable fault, and when the division into compartments now common will be held to be no better than a delusion and a snare.

The protection given to the regular ships-of-war by side armour, or by a protecting deck, at or near the water-line, will probably become a definite and indispensable feature in them, and may, perhaps, be their only distinguishing characteristic, apart from their outfit and equipment.

If this should prove to be the issue of events, their course will have been very indirect. In the ships-of-war of the last century no attempt was made to employ armour on the sides or to prevent the passage of projectiles and water into the holds by means of a protecting deck. There was a deck just below the water-line, but it had no protective qualities. It served, among other things, to furnish passage ways in action for the carpenter and his crew to get at the inner side of the wooden walls of the ship at and near the water-line, so that when shot entered there the holes might be immediately plugged. When screw propulsion was introduced into these ships, and it was found practicable to keep the engines and boilers under water, it would have been possible to place a deck over the machinery and beneath 'the water, which would have greatly added to the security of the engines, boilers, and magazines. The space above this deck might also have been so subdivided into compartments as to have protected the buoyancy and stability of the ship against the immediately fatal results of the invasion of water. The protection of the buoyancy and stability by these means would not have been absolute, in the sense of making the ship safe, but it would have been of the utmost value as compared with ships, otherwise similar, but having no such protection.

Thirty years passed between the date when screw-propeller engines were placed beneath the water-level in ships of war and that at which a committee on designs, under the presidency of Lord Dufferin, proposed to place such a covering deck over them, or to construct a water-line raft-body. The proposal of the main body of the committee was to associate such a raft-deck for the protection of the buoyancy and stability of the ship against artillery with a central armoured citadel. That of the minority was to suppress the armour in the region of the water-line entirely, and to protect buoyancy, stability, machinery, and magazines by a raft-deck alone. In 1873 the plan as indicated by the main body of the committee was put into practice nearly simultaneously in the “Duilio” and “Dandolo” in Italy and in the “Inflexible” in England. In 1878 the system as conceived in principle by the minority of the committee of 1871, although not in the manner they recommended, was adopted in much smaller vessels in the British navy. A raft-deck was introduced into the “Comus” class of corvettes of 2,380 tons displacement, a class which was regarded as unarmoured. Since that date the raft-deck has been adopted in a more or less complete form in nearly all classes of unarmoured ships in the English navy. So it has come about that, out of some 850 unarmoured ships of war built and building in Europe, 47 have such protecting raft-decks. Of these 32 are English. There can be no doubt that all unarmoured ships of war will eventually be protected in this manner. The number of so-called ironclads built and building in Europe is 270. Of these, 34 are based on the recommendation of the committee on designs; 18 of them are English. There are six other English ships with central citadels and under-water protecting decks, built more than twenty years ago, but the raft-body principle is absent in them.

If the passage from the steam line-of-battle ship of 1840-1860 to the “Admiral’’class of 1884 had been made under the guidance of the principles of the committee of 1871, European nations would not find themselves possessed of large fighting ships covered from end to end, or over large areas of their sides, with thin armour, penetrable to a very large proportion of the guns brought against them. But the sailors of 1854-1860 did not take the view that buoyancy and stability, and machinery and magazines, were the vital parts, needing defence by armour or by a raft-deck. They dreaded the effects of shell exploding between decks, setting fire to the ships, and converting the decks, crowded with men, into slaughter-houses. Their demand was, “Keep out the shells.” So it came about that iron armour-plates, thick enough to keep out the most powerful shell of the time, were worked upon the sides of the ships, and the guns were fought through ports cut in this armour. This feeling was so strong that the English Admiralty built the “Hector” and “Valiant” with armoured batteries overlapping by many feet at each end the armour beneath them, which protected the buoyancy, stability, machinery, and magazines. Guns increased in power, and the armour was gradually thickened to resist them, until from 4 ½ inches of armour, through which broadside ports were cut, 9 inches and 10 inches were reached. But this thickening of the armour had so reduced the possible number of the guns in a ship of moderate size, and the guns required for breaching such armour had so increased in weight, that the broadside ship had to give way to the turret or barbette ship, in which about four such guns were all that could be carried, and these had to be worked on turn-tables in or near the central line of the ship.

The point now reaςhed in all navies is that the broadside ironclad with ports cut through an armoured side, as invented in France by M. Dupuy de Lôme, and copied by every power, is obsolete. Guns must be worked singly or in pairs on revolving turn-tables, each turn-table being surrounded by an armoured tower, forming the loading chamber or protecting the mechanism. The side armour protecting the buoyancy, stability, machinery, and magazines, although not introduced for that purpose originally, is retained in France for very large ships, is given up in Italy in favour of a raft-body, and is retained partially in England and Germany in conjunction with a raft-body.

The use of armour has arrested the development of the shell. But it is not inconceivable that its abandonment in front of the long batteries of guns in the French and Italian ships will invite shell attack, and make existence in such batteries, if they are at all crowded, once more intolerable. It remains to be seen whether in that case exposure will be accepted, or a new demand made for armour, at least against the magazine gun and the quick-firing gun. If exposure is accepted, it will be on the ground that the number of men at the guns is now very few, that the gun positions are numerous and the fire rapid, and that, if the guns had once more to be fought through ports in armour, the number of gun positions would be reduced, and the fragments of their own walls, when struck by heavy projectiles, would be more damaging than the projectiles of the enemy.

Internal armour for the protection of the heavy armour-breaching guns must be retained so long as such guns are used, and if they were abandoned an enemy could cover himself with armour invulnerable to light artillery. This the French attempted to do in inaugurating the system. They have been driven from it by the growth of the gun. Abandon the heavy gun, and complete armour-plating might again be adopted.

We must conclude that the buoyancy, stability, machinery, and magazines must be protected as far as possible against fatal damage from a single blow of these armour-breaching guns. The tendency will be to come to the lightest form of such protection. That lightest form appears to be a protecting deck a little above the water-level throughout the greatest part of its surface, but sloping down at the sides and at the ends, so as to meet the side walls of the ship under the water-line. However the armour is arranged (apart from a complete covering with invulnerable plating), —whether as a belt with its upper edge 3 feet out of the water, as in the French ships; as a central armoured citadel and a raft-body at the ends, like the English and German ships; or as a raft-body throughout, like the Italian ships,—shot holes in action will admit water and gradually reduce the necessary stability of the ship. In the French ships the assistance of the unarmoured upper parts is as necessary to prevent them from upsetting in anything but smooth water as is the assistance of the unarmoured raft ends in the English and German ships. In the intact condition the English ships have far greater stability than those of France. In the English ships a reserve of stability is provided, against the contingency of loss by injuries in action. In the French ships no more is provided than is required for the intact condition. The French have not accepted the position taken up in England that much greater initial stability may be given to heavily-armoured broad ships than is usually given, without causing heavy rolling. Nor have they accepted the further incontrovertible truth that the free passage of water in the raft-body from side to side of the ship in rolling is rapidly effective in quelling the motion and bringing the ship to rest in the upright position.

Propulsion.

The propulsion of ships by sails differs from the drifting of bodies in the air before the wind in a roost important respect. Ships may drift or sail in the direct course of the wind, and they will then differ from air-borne bodies only in the comparative slowness imposed by the resistance of the water. Ships having the same length as breadth, or rather opposing the same form and area to side progress as to forward progress could never do other than sail before the wind. No disposition of canvas could make them deviate to the right or left of their course to leeward. But by an alteration of form giving them greater length than breadth, and greater resistance to motion sideways than to motion endwise, they came to possess the power of being able not only to sail to the right or left of the course of the wind, before the wind, but also to sail towards the wind. The wind can be made to impel them towards the point from which it is blowing by means of the lengthened form acted on by the resistance of the water.

Motion directly towards the wind cannot be maintained, but by sailing obliquely towards it first to one side and then to the other progress is made in advance, and the vessel “beats to windward.” The action is like that which would be required to blow a railway car to the eastward by the action of an easterly wind. If the line of rails were due east and west, and the wind were always direct from the east, the thing could not be done. But with a wind to the south or north of east, by setting a sail in the car so that its surface lies between the course of the wind and the direction of the rails, it would then receive the impulse of the wind on its back and would drive the car forwards. There would be a large part of the force of the wind ineffective because of the obliquity of the sail; and of the part which is effective a large portion would be tending to force the car against the rails sideways, but there would be progression to windward. In the case of the ship the resistance to side motion is due to the unsuitability of the proportions and form for progress in that direction as compared with progress ahead, but still there is motion transversely to the line of keel. This motion is called leeway. As the ship moves to leeward and ahead simultaneously there is a point of balance of the forces of the fluid against the immersed body—a centre of fluid pressure. The object of the constructor is to place the masts in the ship in such positions that the centre of pressure of wind upon the sails shall fall a little behind or astern of this centre of resistance of the fluid. In that case there is a tendency in the ship to turn round under the action of these two forces, and to turn with her head towards the wind. This tendency is corrected by the action of the rudder. If the tendency to turn were the other way, although that could also be corrected by the rudder, yet there would be danger of the wind overcoming the rudder action in squalls, and the ship would then come broadside to the wind. In that case, while she might have been quite capable of bearing the pressure of the wind blowing obliquely upon her sails, she might have her sails blown away, or her masts broken, or be herself capsized by the direct impulsion of the wind upon the sail and upon the hull of the ship.

Many examples of disposition of sails might be given. Their disposition is always made to satisfy the conditions that as much sail as possible is required, but if the vessel is small it must be capable of being instantly let go in a squall, or when the wind is gusty. Otherwise, where it cannot be readily let go, its area should be capable of reduction in squally weather, still retaining its efficiency, so that no pressure of the wind should be capable of upsetting the ship. If a sudden violent squall should strike the ship she should find relief, not by a large inclination, but by the blowing away of the sails out of the bolt-ropes, or the carrying away of the masts. One or other of these must of course happen if the area of canvas and the strength of the sails and of the spars are so proportioned at the moment the squall strikes the ship as to be less than the resistance offered by the stability of the ship to a large inclination. Ships are sometimes, when struck by a squall, blown over on to their sides, the sails being in the water. If the sails or spars are then cut away or otherwise got rid of the ship may right herself.

In the Transactions of the Institution of Naval Architects for 1881, Mr W. H. White says :—

“Any investigation of the behaviour of sailing ships at sea must take account of the conditions belonging to the discussion of their rolling when no sail is set, and must superpose upon those conditions the other and no less difficult conditions relating to the action of the wind upon the sails, the influence of heaving motions upon the stability, and the steadying effect of sail-spread.

“It may fairly be assumed that the labours of the late Mr W. Froude have made it possible to predict, with close approximation to truth, the behaviour of a ship whose qualities are known and which has no sails set, when rolling among waves of any assumed dimensions. By a happy combination of experimental investigation and mathematical procedure, Mr Fronde succeeded in tracing the motion from instant to instant, and checked the results thus obtained by comparison with the actual observations made in a sea-way on the behaviour of the ' Devastation.’ The details of his method, and examples of its application, will be found in the Transactions for 1875, and in the appendix to the report of the ‘Inflexible’ committee.

“The conclusion I have reached, after a careful study of the subject, is that we need very considerable extensions of our knowledge of the laws of wind-pressure before more exact investigations will be possible so as to enable us to pronounce upon the safety or danger of a sailing ship. Nor must it be overlooked that sailing ships are not to be treated as machines worked under certain fixed conditions. Their safety depends at least as much upon seamanship and skilful management as upon the qualities with which they are endowed by their designers. Moreover, it is idle to pretend that, in determining what sail-spread can be safely given to a ship, the naval architect proceeds in accordance with exact or purely scientific methods. He is largely influenced by the results of experience with other ships, and thus proceeds by comparison rather than by direct investigation from first principles. Certain scientific methods are employed, of course, in making these comparisons. For example, the righting moment at different angles of inclination is usually compared with the corresponding ‘sail-moment’; but even here certain assumptions have to be made as to the amount of sail to be reckoned in the calculation, and as to the effective wind-pressure per unit of sail-area. Between ship and ship these assumptions are unobjectionable, but they are not therefore to be regarded as strictly true.

“The calculations of curves of stability and the determination of the ranges of stability for ships form important extensions of earlier practice. But, even when possessed of this additional information, the naval architect must resort to experience in order to appreciate fairly the influence of seamanship and the relative manageability of ships and sails of different sizes. There can be no question but that a good range and large area of a curve of stability denote conditions very favourable to the safety of a ship against capsizing. But, in practice, it frequently happens that such favourable conditions can scarcely be secured in association with other important qualities, and a comparatively moderate range and area of the curve of stability have to be considered when the designer attempts to decide whether sufficient stability has been provided. Under these circumstances experience is of the greatest value; a priori reasoning cannot take the place of experience, because (as remarked above) the worst combination of circumstances cannot be fixed, and because some important conditions in the problem are yet unsettled. Certain arbitrary standards may be set up, and ships may be pronounced safe or unsafe; but this is no solution of the problem. There are classes of ships in existence which have been navigated in all weathers, under sail, and in all parts of the world, which might be pronounced unsafe if tested by some of the standards that have been proposed; but the fact that not a single vessel of that class has been capsized or lost at sea during many years will probably be accepted, in most quarters, as sufficient evidence of the seaworthiness of these classes, and as an indication of the doubtful authority of the proposed standards.”

For the different kinds of sails, and for sailmaking, see Sail.

The “Comet” was the first steam-vessel built in Europe that plied with success in any river or open sea. She was built in Scotland in 1811-12 for Mr Henry Bell, of Helensburgh, having been designed as well as built by Mr John Wood, at Port-Glasgow. The little vessel was 42 feet long and 11 feet wide. Her engine was of about four horse-power, with a single vertical cylinder. She made her first voyage in January 1812, and plied regularly between Glasgow and Greenock at about 5 miles an hour. There had been an earlier commercial success than this with a steam vessel in the United States, for a steamer called the “Clermont” was built in 1807, and plied successfully on the Hudson River. This boat, built for Fulton, was engined by the English firm of Boulton & Watt. The reason for this choice of engineers by Fulton appears to have been that Fulton had seen a still earlier steamboat for towing in canals, also built in Scotland, in 1801, for Lord Dundas, and having an engine on Watt’s doubleacting principle, working by means of a connecting rod and crank and single stern wheel. This vessel, the “Charlotte Dundas,” was successful so far as propulsion was concerned, but was not regularly employed because of the destructive effects of the propeller upon the banks of the canals. The engine of the canal boat was made by Mr William Symington, and he had previously made a marine engine for Mr Patrick Miller, of Dalswinton, Dumfriesshire. This last-named engine, made in Edinburgh in 1788, marks, it is said, the first really satisfactory attempt at steam navigation in the world. It was employed to drive two central paddle-wheels in a twin pleasure-boat (a sort of “Castalia”) on Dalswinton Loch. The cylinders were only 4 inches in diameter, but a speed of

5 miles an hour was attained in a boat 25 feet long and 7 feet broad. The first steam vessel built in a royal dockyard was also called the “Comet.” She appears to have been built about the year 1822, and was engined by Boulton & Watt. This ship had two engines of forty horse-power each, to be worked in pairs on the plan understood to have been introduced by the same firm in 1814. In 1838 the “Sirius” and “Great Western” commenced the regular Atlantic passage under steam. The latter vessel, proposed by I. K. Brunel, and engined by Maudslay Sons

& Field, made the passage at about 8 or 9 knots per hour. One year earlier (1837) Captain Ericsson, a scientific veteran who is still among us (1886), towed the Admiralty barge with their lordships on board from Somerset House to Blackwall and back at the rate of 10 miles an hour in a small steam vessel driven by a screw.

The screw did not come rapidly into favour with the Admiralty, and it was not until 1842 that they first became possessed of a screw vessel. This vessel, first called the “Mermaid” and afterwards the “Dwarf,” was designed and built by the late Mr Ditchbum, and engined by Messrs Rennie. In 1841—3 the “Rattler,” the first ship-of-war propelled by a screw, was built for and by the Admiralty under the general superintendence of Brunel, who was also superintending at the same time the construction of the “Great Britain,” built of iron. The engines of the “Rattler,” of 200 nominal horse-power, were made by Messrs Maudslay. They were constructed, like the paddle-wheel engines of that day, with vertical cylinders and overhead crankshaft, with wheel gearing to give the required speed to the screw. The next screw engines made for the Royal Navy were those of the “Amphion,” 300 nominal horse-power, made in 1844 by Miller and Ravenhill. In these the cylinders took the horizontal position, and they became the type of screw engines in general use. This ship had a screw-well and hoisting gear for the screw. In 1845 the importance of the screw propeller for ships of war became fully recognized, and designs and tenders were invited from all the principal marine engineers in the kingdom. The Government of that day then took the bold step of ordering at once nineteen sets of screw engines. Six of these had wheel gearing; in all the rest the engines were direct-acting. The steam pressure in the boilers was from 5 to 10 lb only above the atmosphere, and if the engines indicated twice the nominal power it was considered a good performance. The most successful engines were those of the “Arrogant” and “Encounter” of Messrs Penn. They had a higher speed of piston than the others, and the air-pumps were worked direct from the pistons, and had the same length of stroke. These engines developed more power for a given amount of weight than other engines of their day, and were the forerunners of the many excellent engines on the double-trunk plan made by this firm for the navy. The engines with wheel-gearing for the screws were heavier, occupied more space, and were not so successful as the others, and no more of that description were ordered for the British navy.

Up to 1860 neither surface-condensers nor superheaters were used in the navy. The consumption of fuel was about 4 1/2 lb per one horse-power per hour. In that year (1860) three ships, the “Arethusa,” “Octavia,” and “Constance,” were fitted respectively by Messrs Penn, Messrs Maudslay, and Messrs Elder, with engines of large cylinder capacity to admit of great expansion, with surface-condensers aud superheaters to the boilers. Those of the “Arethusa” were double-trunk, with two cylinders; those of the “Octavia” were three-cylinder engines; and those of the “Constance” were compound engines with six cylinders; the first two were worked with steam of 25 lb pressure per square inch, and the last with steam of 32 lb pressure. All these engines gave good results as to economy of fuel, but those of the “Constance” were the best, giving one indicated horse-power with 2 ½ lb of fuel. But the engines of the “Constance” were excessively complicated and heavy. They weighed, including water in boilers and fittings, about 5 ½ cwts. per maximum indicated horse-power, whereas ordinary engines varied between 3 ½ and 4 ¾ cwts.

For the next ten years engines with low-pressure steam, surface-condensers, and large cylinder capacity were employed almost exclusively in the ships of the Royal Navy. A few compound engines, with steam of 30 lb pressure, were used in this period with good results as to economy, but they gave trouble in some of the working parts. Compound engines, with high-pressure steam (55 lb), were first used in the Royal Navy in 1867, on Messrs Maudslay's plan, in the “Sirius.” These have been very successful. In the Royal Navy as well as in the mercantile marine, the compound engine is now generally adopted. They have been made rather heavier than the engines which immediately preceded them, but they are about 25 per cent. more economical in fuel, and, taking a total weight of machinery and fuel together, there is from 15 to 20 per cent. gain in the distance run with a given weight

Wrought-iron is largely used in the framing in the place of castiron, and hollow propeller shafts made of Whitworth steel. By these means the weight is being reduced, and it is to be hoped that a still further reduction may yet be made by the use of high-class materials in the engines and steel in the boilers.

Mr Thornycroft, of Chiswick, and others, by means of high rate of revolution, forced combustion, and the judicious use of steel, have obtained as much as 455 indicated horse-power with a total weight of machinery of 11 ¾ tons, including water in boilers. The ordinary weight of a seagoing marine engine of large size, with economical consumption of fuel, excepting a few of very recent construction, would be six or seven times as great. By closing in the stokeholes and employing fans to create a pressure of air in them capable of sustaining from one to two inches of water in the gauges the consumption of coal per square foot of fire-grate per hour may be raised to 130 lb and upwards. The indicated horse-power which can be obtained in ordinary cases with the steam-blast in the chimney to quicken consumption does not exceed ten. But by the forced draft above described it can be raised with ordinary boilers to 17 to 18 indicated horse-power per square foot of firegrate. In torpedo boats with locomotive boilers over 28 horsepower per foot of fire-grate is attainable.

The following observations on efficiency are taken from the work of Mr Sennett on The Marine Steam Engine :—

“In every machine there are always certain causes acting that produce waste of work, so that the whole work done by the machine is not usefully employed, some of it being exerted in overcoming the friction of the mechanism, and some wasted in various other ways. The fraction representing the ratio that the useful work done bears to the total power expended by the machine is called the efficiency of the machine; or—

. Useful work done.

Efficiency = — i ——

Total power expended.

In the marine steam engine, in which the useful work is measured by its propelling effect on the ship, there are four successive stages, in each of which a portion of the initial energy is wasted, and these four causes all tend to decrease the efficiency of the engine as a whole.

“In the first place, only a portion of the heat yielded by the combustion of the coal in the furnaces is communicated to the water in the boiler, the remainder being wasted in various ways. The fraction of the total heat evolved by the combustion of the coal, that is, transmitted to the water in the boiler, is in ordinary cases not more than from 5/10 to 7/10. This fraction is called the efficiency of the boiler.

“Secondly, the steam, after leaving the boiler, has to perform mechanical work on the piston of the engine; but this work, in consequence of the narrow limits of temperature between which the engine is worked, is only a small fraction of the total heat contained in the steam—say from 1/5 to 1/20, according to the kind of engine and rate of expansion employed. This fraction, representing the ratio of the mechanical work done by the steam to the total amount of heat contained in it, is called the efficiency of the steam.

“Thirdly, in the engine itself a part of the work actually performed by the steam on the pistons is wasted in overcoming the friction oi' the working parts of the machinery and in working the pumps, &c. The remainder is turned into useful work in driving the propeller. The fraction representing the ratio that this useful work bears to the total power exerted by the pistons is called the efficiency of the mechanism.

“Fourthly, the propeller, in addition to driving the ship ahead, expends some of the power transmitted to it in agitating and churning the water in which it acts, and the work thus performed is wasted,—the only useful work being that employed in overcoming the resistance of the ship and driving her ahead. The ratio of this useful work to the total power expended by tho propeller is called the efficiency of the propeller.

“The resultant efficiency of the marine steam engine is made up of the four efficiencies just stated, and is given by the product of the four factors representing respectively the efficiencies of the boiler, the steam, the mechanism, and the propeller. Any improvement in the efficiency of tho marine steam engine, and, consequently, in the economy of its performance, is therefore due to an increase in one or more of these elements.”

Under Steam Engine will be found a discussion of the first three of the efficiencies enumerated above. Propulsion and propellers have to be considered here.

“The principle upon which nearly all marine propellers work,” says Mr Sydney Barnaby, “is the projection of a mass of water in a direction opposite to that of tho required motion of the vessel. When a vessel is in motion at a regular speed the reaction of the mass of water projected backwards by the propeller is exactly equal to the resistance experienced by the vessel. When it is clearly understood that propulsion is obtained by the reaction of a mass of water projected sternwards with a velocity relative to smooth water, the absurdity is at once seen of attempting to get a propeller to work without slip. If there is no slip there is no resultant propelling reaction except in the limiting case where the mass of water acted upon is infinite. The whole problem therefore resolves itself into this—What is the best proportion between the mass of water thrown astern and the velocity with which it is projected, that is, if the screw propeller is under consideration, the ratio between its diameter and its pitch ?”

“There are four different kinds of propellers apart from sails— the oar, the paddle-wheel, the screw, and tho water jet.

“The first and oldest of them—the oar—may be used in two ways. The action may be intermittent, as in rowing, when water is driven astern during half the stroke and the instrument brought back above the water; or its action may be continuous, as in sculling. When used as in rowing it is exactly analogous to a paddle-wheel, while the action of the scull closely resembles that of the screw. It is supposed that in the ancient galleys, which were propelled by a large number of oars in several tiers or banks, the oars hung vertically and worked inwards and outwards with a sculling action. They were not removed from the water, but served as props when the vessel was aground. The oars were always propelling the vessel, in both parts of the stroke. The rowers generally sat with their faces outwards and forwards. There was great overhang of the sides to allow of several tiers of rowers one above another. The oar as used for rowing is a very efficient instrument. To obtain the maximum efficiency out of it a constant pressure should be maintained upon the oar, so that the water is started gradually from rest, and the acceleration uniformly increased throughout the whole of the stroke. A glance at a university crew will show that the stroke is kept up with a uniform pressure and without any jerk.”

Speaking of the screw propeller, Mr S. Barnaby says:—“The speed with which water can follow up the blades of a screw depends upon the head of water over it, but when the immersion is suffi- cient to exclude air a head of water equivalent to 30 feet is supplied by the atmosphere, as has been pointed out by Prof. Osborne Reynolds. Experiments on the model of the Thornycroft screw have shown that the efficiency, which is as much as 70 per cent. when properly immersed, falls to about 50 per cent. when breaking the surface of the water. As a result of a change from a diameter of 5 feet 10 inches to 4 feet 6 inches the speed of the first-class torpedo boat was raised from 18 to 20 knots, other conditions remaining the same.

"There is no doubt that the stern is the best position for the screw. As a vessel passes through the water the friction imparts motion to the layer of water rubbing against the side. This layer increases in thickness towards the stern, so that, after the vessel has passed through, a considerable quantity of water is left with a motion in the same direction as the vessel. If the screw works in this water it is able to recover some of the energy which has been expended by the ship in giving it motion. The speed of this water, which Rankine estimates may be as much as one-tenth of the speed of the vessel, does not depend upon the form but upon the nature and extent of the surface. As it is a necessity that there should be such a wake, it is a distinct advantage to place the propeller in it and allow it to utilize as much as possible of the energy it finds there. It is important not to confound this watet, which has had motion given to it by the side and bottom of the ship, with the wave of replacement, that is, the water filling in behind the ship. It should be the aim to interfere as little as possible with this motion, as such interference augments the resistance of the ship very considerably, even in well-formed ships. The propeller should therefore be kept as far away from the stern as possible.

“In the small high-speed stern launches the propeller has been kept outside the rudder, with advantage to the speed. What is required is that before reaching the screw the water shall have given out upon the stern of the ship the energy put into it by the bow. If a screw propeller is placed behind a bluff stern so that its supply of water is imperfect it will draw in water at the centre of the driving face, and throw it off round the tips of the blades, like a centrifugal pump, thus producing a loss of pressure upon the stern of the vessel. For very high speed vessels several propellers would enable the weight of the machinery to be kept down. The weight of an engine of a given type per indicated horse-power varies inversely as the number of revolutions per minute; that is, the greater the number of revolutions the less the weight per indicated horse-power.

“There is a certain quantity of work which must be lost with any propeller, and it is equal to the actual energy of the discharged water moving astern of the propeller with a velocity relative to still water. As this energy varies as the weight multiplied by the square of the velocity, if we double the quantity of water acted upon we double the loss from this cause, but if we double the velocity with which the water is discharged we increase the loss fourfold. This shows the advantage of acting upon a large column of water, and leaving it with as small a speed as possible relative to still water. For this reason the screw is a more efficient instrument than a paddle-wheel, and the jet propeller, with its small area of jet, is so much inferior to the screw. From the above considerations it would appeal that the larger the diameter of a screw and the smaller the slip the greater the efficiency would be. There is, however, another element of loss which has to be considered, which imposes a limit to the size of a screw in order to obtain the best efficiency. This element is the friction of the screw blades. How large the effect of this element may be is shown by the case of H.M. S. ‘Iris.’ This ship was originally fitted with two four-bladed propellers, 18 feet in diameter, and with 18 feet pitch or velocity of advance per revolution. She obtained a speed with these propellers of 15 ¾ knots with an expenditure of 6369 horse-power. Two blades were then taken from each propeller, reducing the total number from eight to four. The indicated horse-power then required for the same speed was 4369, or two thousand less horse-power. This amount had been lost in driving the four additional blades.”

" The causes of loss of work incidental to propellers of different kinds may be summed up as follows:—(1) Suddenness of change from velocity of feed to velocity of discharge. Propellers which suffer from this cause are the radial paddle-wheel and the common uniform pitch screw; while those which in varying degree avoid it are the gaining pitch screw, the feathering paddle-wheel, Ruthven’s form of centrifugal pump, and the oar. (2) Transverse motion impressed on the water. Propellers which lose in efficiency from this cause are ordinary screw-propellers, which impart rotary motion, radial wheels, which give both downward and upward motion on entering and leaving the water, and oars, which impart outward and inward motion at the commencement and end of the stroke respectively. This loss is greatly reduced in the guide-propeller, as the guides take the rotary motion out of the water and utilize it in so doing. (3) Waste of energy of the feed water. This is experienced in the jet propeller as generally applied.”

The present condition of the case of screw steamship propulsion appears, according to Mr Froude’s estimate, to be that, calling the effective horse-power (that is, the power due to the net resistance) 100, then at the highest speeds the horse-power required to overcome the induced negative pressure under the stern consequent on the thrust of the screw is 40 more; the friction of the screw in the water is 10 more; the friction in the machinery 67 more; and air-pump resistance perhaps 18 more; add to this 23 for slip of screw, and we find that, in addition to the power required to overcome the net resistance = 100, we need 40 + 10 + 67 + 18 + 23, making in all 258; i . e ., at maximum speeds the indicated power of the engines needs to be more than two-and-a-half times that which is directly effective in propulsion. (N. B.)

Boatbuilding.

The foregoing article may be supplemented by a brief account of boatbuilding. The distinction between this and shipbuilding is not of a marked character and cannot be sharply defined. But for all practical purposes the builder of a vessel without a deck, or but partially decked, and propelled partly by sails and partly by oars, or wholly by oars, may be defined as a boatbuilder.

The boats in general use at present may be classified as racing boats, pleasure boats, or boats used for commercial purposes. Racing boats (compare Rowing) are generally built of mahogany, and are the most perfect specimens of the boatbuilder’s art. The outrigger sculling boat measures from 30 to 35 feet long, 12 to 14 inches in breadth, and 9 inches in depth, weighing only from 35 to 45 lb, and the eight-oared outrigger, being from 55 to 65 feet long by 2 feet 2 inches to 2 feet 5 inches in breadth, weighs about 300 lb. Pleasure boats vary in form and dimensions, from the 15-feet rowing boat used on the sea-coast to the gondola type found principally on the canals of Venice and used occasionally on the Thames, &c., for ceremonial pageants. Boats used for commercial purposes embrace fishing, canal, and ships’ boats. Fishing boats (compare Fisheries) are gradually passing from the sphere of the boatbuilder to that of the shipbuilder,—the open boats of former years being in many cases replaced by large, strong, decked craft more able to withstand the gales of the British coasts. Canal boats are generally long, narrow, and shallow, from 50 to 70 feet long by 8 to 10 feet in breadth, and from 4 to 5 feet in depth. All sea-going vessels are required by statute to be provided with boats fully equipped for use, not fewer in number nor less in their cubical contents than what is specified for the class to which the ship belongs. The boats vary considerably in form and dimensions as well as in material and construction, according to the service intended. The number of boats a passenger steamer of 1000 tons and upwards is required to carry is six or seven, according to the dimensions of the boats. In either case two of the largest boats must be fitted as lifeboats. If the smaller number is carried, the set will consist of two lifeboats, one launch, two cutters or pinnaces, and one gig.

Lifeboats are built both ends alike, having a sheer or rise from midships towards stem and stern of 1/2 inch to 3/4 inch per foot of length. They have air-cases of copper or yellow metal fitted in the ends and along the sides of the boat, of sufficient capacity to give each person carried in the boat one and a half cubic feet of strong enclosed air-space (compare vol. xiv. p. 570). Cutters are similar in form but of smaller dimensions than lifeboats; pinnaces are about the same dimensions as cutters, but have square sterns. Gigs are of lighter construction and finer form than pinnaces. A service boat called a dingy is also carried, for the conveyance of light stores between the shore and the vessel. Boats, when carried so close to the funnel of a steamer as to be injuriously affected by the heat therefrom, have of late years been built of zinc, iron, or steel. Those built of steel have plates 1/16 inch thick and galvanized, the keel, stem, stern, and deadwood knees being of wood, to which the plating is attached.

The following is an outline of the method of construction. The designer lays down on paper the lines and body-plan of the craft, which are afterwards traced full size on the floor of the drawing-loft. From these full-sized sections moulds are made. The stem and stern posts, having been cut out to the shape designed, are tenoned into mortices in the keel. Two knees overlap, and bind the stem and stern posts to the keel, and are bolted with through bolts and clenched outside over a ring or washer. A stout batten of wood is then nailed between the stem and sternpost heads to connect them together, and a line is then stretched from stem to sternpost to represent the water-line. The keel, stem, and stern posts being in position on the stocks, the stem and stern posts are then plumbed and secured by stays of wood. The rabbets in the keel, stem, and stem posts are then cut out with a chisel, after which the moulds are put into their proper places, plumbed with the water-line, and kept in position by stays. The planking is then proceeded with, strake after strake, and when the boat is planked up to the top strake the floors and timbers are put in. The floor extends across the keel and up to the turn of the bilge. They are fastened through the keel with copper or yellow metal bolts and to the planking with copper nails. The timbers generally are about 1 inch by ¾ inch, and are sawn out of a clean piece of American elm, then planed and rounded. After being steamed they are fitted into the boat, and as soon as each is in position, and before it cools, it is nailed fast with copper nails. The gunwale is next fitted, a piece of American elm about 2 inches square; a breast-hook is fitted forward, binding the gunwale, top strake, stern, and apron together; and aft the gunwale and top strake are secured to the transom by either a wooden or iron knee. A waring or stringer, about 3 inches by ¾ inch, of American elm, is then fitted on both sides of the boat, about 8 to 9 inches below the gunwale, on the top of which the thwarts or seats rest The thwarts are secured by knees, which are fastened with clench bolts through the gunwale and top strake and also through the thwart and knee. The boat generally receives three coats of ρaiut and is then ready for service.

The following are the dimensions of boats in the British merchant service :— [table]